© Horizon Scientific Press. Offprints from www.cimb.org
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Curr. Issues Mol. Biol. 9: 21–40. Online journal at www.cimb.org
Molecular Diagnostics of Medically Important
Beverley Cherie Millar1, Jiru Xu2, and John
1Northern Ireland Public Health Laboratory, Department
of Bacteriology, Belfast City Hospital, Belfast, Northern
Ireland, BT9 7AD, UK
2Northern Ireland Public Health Laboratory, Department
of Bacteriology, Belfast City Hospital, Belfast, Northern
Ireland, BT9 7AD, UK, and Department of Pathogenic
Biology, Xian-Jiatong University, Xi’an, The People’s
Republic of China
Infectious diseases are common diseases all over
the world. A recent World Health Organization report
indicated that infectious diseases are now the world’s
biggest killer of children and young adults. Infectious
diseases in non-industrialized countries caused 45% in all
and 63% of death in early childhood. is by. In developed
countries, the emergence of new, rare or already-forgotten
infectious diseases, such as HIV/AIDS, Lyme disease and
tuberculosis, has stimulated public interest and inspired
commitments to surveillance and control. Recently, it is
reported that infectious diseases are responsible for more
than 17 million deaths worldwide each year, most of which
are associated with bacterial infections. Hence, the control
of infectious diseases control is still an important task in
the world. The ability to control such bacterial infections is
largely dependent on the ability to detect these aetiological
agents in the clinical microbiology laboratory.
Diagnostic medical bacteriology consists of two main
components namely identification and typing. Molecular
biology has the potential to revolutionise the way in which
diagnostic tests are delivered in order to optimise care
of the infected patient, whether they occur in hospital
or in the community. Since the discovery of PCR in the
late 1980s, there has been an enormous amount of
research performed which has enabled the introduction
of molecular tests to several areas of routine clinical
microbiology. Molecular biology techniques continue
to evolve rapidly, so it has been problematic for many
laboratories to decide upon which test to introduce before
that technology becomes outdated. However the vast
majority of diagnostic clinical bacteriology laboratories do
not currently employ any form of molecular diagnostics but
the use such technology is becoming more widespread in
both specialized regional laboratories as well as in national
reference laboratories. Presently molecular biology offers
a wide repertoire of techniques and permutations of these
analytical tools, hence this article wishes to explore the
application of these in the diagnostic laboratory setting.
The last ten years of the twentieth century allowed for an
exponential increase in the knowledge of techniques in
molecular biology, following the cellular and protein era
of the 1970s and 1980s. This explosion of technologies
from the primary discipline of molecular biology has had
major consequences and has allowed for significant
developments in many areas of the life sciences,
including bacteriology. Molecular bacteriologists are now
beginning to adopt general molecular biology techniques
to support their particular area of interest. This chapter
aims to examine the current situation with regard to the
application of molecular biology techniques in the area
of medical bacteriology, and is primarily concerned
with the molecular identification of causal agents of
bacterial infections. The chapter also aims at giving a
broad overview of the application of current technology
so that the reader has a more comprehensive overview
of the diversity of techniques that are available, either
as research tools or which may be used in a routine
setting. The requirements of adopting molecular
diagnostics in terms of management, cost, labour and
space will be discussed. This chapter is a presentation
of the development and the current knowledge, literature,
and recommendation about the laboratory diagnosis
of infectious diseases. The focus of this review is on
medically important bacterial infections. The overall aim
of this chapter is to provide an appreciation of the role
molecular diagnostics has in routine clinical microbiology
and how best these techniques can be integrated in order
to enhance the healthcare system.
In 1676, Anton van Leeuwenhoek, a Dutch cloth
merchant and amateur lens grinder, first observed living
microorganisms using his simple microscope, which he
called “animalcules”. He examined “animalcules” in the
environment, including pond water, sick people and
even his own mouth and found that these “animalcules”
existed everywhere. He described and recorded all the
major kinds of microorganisms: protozoa, algae, yeast,
fungi, and bacteria in spherical, rod, and spiral forms. His
discoveries opened up a new world namely the microbial
world and this was the first milestone in the history of
Although the suggestion that disease was caused
by invisible living creatures was made by the Roman
physician Girolamo Fracastoro in 1546, people did not
clearly recognize the role of microorganisms in diseases,
until 1876, after 200 years that Leeuwenhoek found his
little “animalcules”, the German physician, Robert Koch
established his famous “Koch’s postulates” according to
the relationship between Bacillus anthracis and anthrax.
Koch’s postulates include:
22 Millar et al.
a. In order to prove that a certain microbe is the cause
of a certain disease, that same microbe must be
found present in all cases of the disease.
b. This microbe must then be completely separated
from the diseased body and grown outside that body
in a pure culture.
c. This pure culture must be capable of gibing the
disease to healthy animals by inoculating them with
d. The same microbe should then be obtained from the
animals so inoculated, and then grown again in a
pure culture outside the body.
Koch was a great pioneer in medical microbiology
and his postulates are still considered fundamental
to bacteriology even today. Communicable diseases
can occur in populations and cause epidemics or even
pandemic problems. Some outbreak diseases are so
serious that they can cause hundreds, thousands, even
millions of deaths, in epidemic or pandemic proportions.
For example, bubonic plague, caused by Yersinia pestis,
originally spread from Asia and was carried by rat-fleas
via the ports of the Black Sea to Europe, which caused 42
million deaths, 25 million in Europe, in just less than five
years between 1347 and 1352, reducing the population
of Europe to 50 million. The scourge of tuberculosis was
about to appear. Bunyon’s epithet, “The Captain of the
Men of Death”, tells of the overwhelming fear that the
disease was associated with, even until today. Until the
present era, it was recognized and feared as one of the
most common and most serious hazards of life, one from
which escape was almost impossible. At the same time,
the well-known Renaissance physician, Jean François
Fernel was performing some early and important studies
on the circulatory system. In Fernel’s book Medicini, he
describes descriptions of several pathological conditions,
including what is now believed to be some of the earliest
published accounts of endocarditis.
Comparing conventional detection methods of which
have been developing over a century, molecular detection
methods are relatively very young, with a history of
approximately no more than 20 years old. Although
deoxyribonucleic acid, or DNA, was discovered in the
late 1860s, it was not used until the restriction enzyme
and the recombinant DNA techniques were discovered
in the 1970s. During this time, many scientists worked
diligently to find the profound mystery of DNA. To trace
the development of molecular detection methods, we
should remember some pioneers and their discoveries.
In 1869, Johann Friedrich Miescher, a Swiss
physician, discovered a weakly acidic substance of
unknown function in the nuclei of human white blood cells,
this substance, which was later named, deoxyribonucleic
acid, or DNA. The substance was largely ignored for
nearly a century because it seemed too simple to serve
any significant purpose. This view changed dramatically
in 1949, when Erwin Chargaff, a biochemist, reported that
DNA composition was species specific; that is, that the
amount of DNA and its nitrogenous bases varied from one
species to another. In addition, Chargaff founds that the
amount of adenine equalled the amount of thymine, and
the amount of guanine equalled the amount of cytosine
in DNA, from every species. During this time scientists
discovered that chromosomes, which were known to carry
hereditary information, consisted of DNA and proteins.
In 1928, Franklin Griffith, a British medical officer,
discovered that genetic information could be transferred
from heat-killed bacteria cells to viable organisms.
This phenomenon, called transformation, provided the
first evidence that the genetic material is a heat-stable
chemical. In 1944, Oswald Avery, a Canadian physician
and bacteriologist, and his colleagues McCarty and Colin
MacLeod, identified Griffith’s transforming agent as DNA.
Experiments conducted throughout the 1940s showed
that DNA actually seemed to be the genetic material.
However, it was still not known what the structure of
DNA was, and how such a molecule could contain all the
information needed to produce a human being or other
living organisms, until 1953, when James Watson and
Francis Crick discovered the molecular structure of DNA.
After building successive scale models of possible DNA
structures, they deduced that it must take the twisted-
ladder shape of a double helix. The sides of the ladder
consist of a “backbone” of sugar and phosphate molecules.
The nitrogen-rich bases, A, T, G and C, form the “rungs” of
the ladder on the inside of the helix. The pair discovered
that base A would only pair with T, while G would only pair
with C. They were awarded the Nobel Prize in Physiology
or Medicine in 1962 for their discovery, shared with
Maurice Wilkins, whose work with Rosalind Franklin on X-
ray crystallography had provided further crucial evidence.
In 1961, François Jacob and Jacques Monod develop a
theory of genetic regulatory mechanisms, showing how,
on a molecular level, certain genes are activated and
suppressed and they were awarded the Nobel Prize in
Physiology or Medicine in 1962 for their contribution.
In 1961 Marshall Nirenberg, a young biochemist at the
National Institute of Arthritic and Metabolic Diseases,
discovered the first “triplet”—a sequence of three bases
of DNA that codes for one of the twenty amino acids that
serve as the building blocks of proteins. Subsequently,
within five years, the entire genetic code was deciphered.
At end of 1960s, almost all about the DNA structures
and functions were understood in theory, but people still
could not get any gene as they wanted or change any
gene as they needed until 1970s when some in important
enzymes were discovered. In 1970, Hamilton Smith,
an American microbiologist, isolated the first restriction
enzyme, an enzyme that cuts DNA at a very specific
nucleotide sequence. Over the next few years, several
more restriction enzymes were isolated. He shared
the Nobel Prize in Physiology or Medicine with Werner
Arber and Daniel Nathans in 1978 for his discovery. In
1972, Paul Berg assembled the first DNA molecules that
combined genes from different organisms. Results of his
experiments represented crucial steps in the subsequent
development of recombinant genetic engineering. In
1980, Paul Berg shared the Nobel Prize in Chemistry with
Walter Gilbert and Frederick Sanger, for “his fundamental
studies of the biochemistry of nucleic acids, with particular
regard to recombinant DNA.” In 1973, Stanley Cohen
and Herbert Boyer combined their efforts to create the
Molecular Diagnostics of Medically Important Bacterial Infections 23
construction of functional organisms that combined and
replicated genetic information from different species. Their
experiments dramatically demonstrated the potential
impact of DNA recombinant engineering on medicine and
pharmacology, industry and agriculture. Walter Gilbert
(with graduate student Allan M. Maxam) and Frederick
Sanger, in 1977, working separately in the United States
and England, developed new techniques for rapid DNA
sequencing. The methods devised by Sanger and Gilbert
made it possible to read the nucleotide sequence for entire
genes, which run from 1,000 to 30,000 bases long. For
discovering these techniques Gilbert and Sanger received
the Albert Lasker Medical Research Award in 1979, and
shared the Nobel Prize in Chemistry in 1980 (Genome
News Network, http://www.genomenewsnetwork.org). In
the 1970s, nucleic acid hybridization methods were mostly
used methods and DNA probes were the powerful tools in
molecular biology, microbiology, virology, genetics, and
forensics etc. Although hybridization methods are highly
specific for detecting targets, they are limited by their
sensitivity. To get more sensitive and specific methods,
Kary Mullis conceived and helped develop polymerase
chain reaction (PCR), a technology for rapidly multiplying
fragments of DNA in 1983 and he was awarded the
Nobel Prize in Chemistry in 1993 for this achievement. In
1985, Saiki and his colleagues first used the new method
to detect patient’s β-globin gene for diagnosis of sickle
anaemia. In 1987, Kwok and colleagues identified human
immunodeficiency virus (HIV) by using PCR method and
this was the first report the application of PCR in clinical
diagnosis infectious disease.
Over the past 20 years, molecular techniques have
been developed very broadly and fast. The nucleic
acid amplification technology has been opened a new
century for microbial detection and identification. At
present, molecular detection methods, especially PCR-
based methods, have become more and more important
detection methods in the clinical diagnosis laboratory
Applications of molecular diagnostic identification
Molecular identification should be considered in
three scenarios, namely (a) for the identification of an
organism already isolated in pure culture, (b) for the
rapid identification of an organism in a diagnostic setting
from clinical specimens or (c) for the identification of an
organism from non-culturable specimens, e.g. culture-
Difficult to identify organisms
Most modern clinical microbiology diagnostic laboratories
rely on a combination of colonial morphology, physiology
and biochemical/serological markers, for their successful
identification either to the genus level or more frequently
to the species level. It is important that organisms are
correctly identified for a number of reasons, including the
correct epidemiological reporting of causal agents in a given
disease state, as well as for infection control purposes.
Sometimes it is argued that physicians may simply accept
the Gram result and corresponding antibiogram in order to
determine the optimum management plan of an infected
patient. This lack of identification ability requires caution,
for the two reasons stated above. Consequently, there is a
need to reliably identify organisms of clinical significance
in a cost effective and timely manner. Most laboratories
today rely on identification through biochemical profiling
with the API-identification schema, as well as with the
BBL-Crystal system, although there are several additional
phenotypic systems, which are commercially available.
Employment of one or a combination of such phenotypic
schemes offers the diagnostic microbiologist a result in
virtually all clinical situations, however, there is a small
number of organisms where such methods are unable
to give a reliable identification, e.g., the non-fermenting
Rapid identification from clinical specimens
Traditional culture may take several days to allow
sufficient growth of an organism so that a positive
identification may be subsequently made. Often in clinical
microbiology, such allowances in time result in patients
being managed empirically, until the culture result is
known, which may allow for the sub-optimal management
of some patients. However, there is a role for molecular
identification techniques from clinical specimens where a
culture would give you a comparable result, but several
days later. Hence, there is a role for such techniques in
outbreak settings, as well as in today’s world of potential
Identification from culture-negative specimens
Since its origins in the late 19th century, bacteriology has
largely been based on the ability to culture organisms of
interest usually under in vitro laboratory conditions. The
forefathers of bacteriology including Pasteur and Koch
were ardent exponents of bacteriological culture and the
affinity between the bacteriologist and laboratory culture
has remained strong for the past 100 years. Indeed, it may
be argued that the historical success of bacteriology has
been a direct result of bacteriological culture, as well as
its widespread adoption throughout the world. Today the
ability to culture bacteria in vitro remains the cornerstone
of this discipline. However, there are several situations
where molecular approaches should be considered
where conventional culture fails to identify the causal
organism due to one or more of the following reasons
including (i) prior antibiotic therapy, e.g. treatment of
acute meningitis with i.v. benzylpenicillin, (ii) where the
organism is fastidious in nature, such as the HACEK
group of organisms in the case of endocarditis, (iii) where
the organism is slow growing, e.g. Mycobacterium spp.,
(iv) where specialized cell culture techniques are required,
e.g. Chlamydia spp. and Coxiella burnetti.
Molecular methods may be included in the diagnostic
laboratory’s diagnostic algorithm,
laboratories to make a rapid and reliable identification.
Several molecular approaches may be adopted to help
with the identification.
Description of molecular techniques employed in
Nucleic acid hybridization is based on the ability of two
single nucleic acid strands that have complementary base
sequences to specifically bond with each other and form
24 Millar et al.
a double-stranded molecule, or duplex or hybrid. The
single-stranded molecules can be RNA or DNA, and the
resultant hybrids formed can be DNA-DNA, RNA-RNA, or
DNA-RNA. Hybridization assays require that one nucleic
acid strand (the probe) originates from an organism of
known identity and the other strand (the target) originates
from an unknown organism to be detected or identified.
The probes are capable of identifying organisms at,
above, and below the species level. Hybridization
reactions can be done using either a solution format or
solid support format. The solid support formats include:
filter hybidizations, sandwich hybridizations, and in
situ hybridizations. Nucleic acid hybridization methods
were developed in the 1970s, and they are still used in
microbial detection and identification today. They are also
the important detection tools in real-time PCR such as
TaqMan and LightCycler platforms.
Polymerase chain reaction (PCR)
Principle of PCR
Polymerase Chain Reaction (PCR) is an enzyme-driven,
primer-mediated, temperature-dependent process for
replicating a specific DNA sequence in vitro. The principle
of PCR is based on the repetitive cycling of three simple
reactions, the conditions of which vary only in the
temperature of incubation. The three simple reactions
1. Denaturing: When the temperature is raised to around
95oC, template DNA double strand is separated to
two single strands.
2. Annealing: When the temperature reduces to approx.
55 oC, two specific oligonucleotide primers bind to the
DNA template complementarily.
3. Extension: When the temperature rises to 72 oC, DNA
polymerase extends the primers at the 3’ terminus
of each primer and synthesizes the complementary
strands along 5’to 3’ terminus of each template DNA
using desoxynucleotides containing in media. After
extension, two single template DNA strands and two
synthesized complementary DNA strands combine
together forming two new double strand DNA copies.
After extension, the reaction will repeat above steps.
Each copy of DNA may then serve as another
template for further amplification. PCR products will
be doubled in each cycle. After n cycles (approx. 30),
the final PCR products will have 2n copies of template
DNA in theory and it just needs few hours.
Specific PCR is the simplest PCR approach of which is
designed for detecting specific target microbes. In specific
PCR, primers are designed complimentary to a known
DNA target and specific for the microbe being assayed.
This is a key point for specific PCR so that the primers
should be so-designed so that they are strictly specific for
the targeted microorganisms. As the result is specific for
the detection of target microbes, this method can be used
as a direct detection and identification method. This is the
most widely used method in the diagnosis of infectious
diseases. Many organisms, such as Mycobacterium
Burkholderia cenocepacia, can be identified by specific
In multiplex PCR, two or more primer pairs are included
in one reaction tube and two or more DNA templates
are targeted simultaneously. This is a relatively simple
molecular way to detect few different bacteria in one PCR
reaction. In multiplex PCR, the primer pairs should be
specific to the target gene and the PCR products should
be in different sizes.
In this approach, genomic template DNA is amplified with
two sets of primers. The first PCR set produces a larger
PCR product than that in second PCR set. The second
PCR set uses the first PCR product as template DNA
to amplify an internal region of DNA during the second
(nested/semi-nested) amplification stage. The primers
in the second PCR set can be different to the first set
(nested) or one of the primers can be the same as the first
set (semi-nested). This method can be used to increase
the sensitivity of detection or to identify the first set PCR
products when the primers in the second PCR reaction
Broad range PCR
Broad range PCR is a very useful approach for detecting
microbes universally. The primers in broad range PCR are
selected from the conserved regions of a particular gene
that is shared by a given taxonomic group. This crucial
important for broad range PCR to select real broad range
primes. As the 16S rRNA gene is found in all bacteria
and contains certain conserved regions of sequence, it
has been mostly used as the broad range PCR target
gene for detecting bacteria. The 23S rRNA is similar with
the 16S rRNA gene and it may also be used to a lesser
extent, as the broad range PCR target gene. Given that,
there is relatively limited information regarding the 23S
rRNA gene, this gene locus gene is not so popular, in
comparison with the 16S rRNA gene. Another region of
the rRNA machinery that is used in broad range PCR is
the inter-spacer (ITS) region between 16S and 23S rRNA
genes. In this broad range PCR approach, the forward
primer is from 16S rRNA gene, and the rewords primer
is form 23S rRNA gene. An example of the arrangement
of the rRNA operons is illustrated with the obligate Gram
+ve intracellular pathogen, Tropheryma whipplei.
1. Nested PCR: After broad range DNA amplification,
species-specific primers are used in tandem with a
second set of DNA amplification primers. The result
can be detected by standard gel electrophoresis.
2. DNA probe hybridization: The broad range PCR
products can be identified by using species-specific
DNA probe hybridization. The probe and the PCR
products are incubated together in a single test tube,
and the binding of probe to the target is measured.
3 DNA enzyme immunoassay (DEIA): In this method,
Molecular Diagnostics of Medically Important Bacterial Infections 25
an anti-dsDNA antibody, particularly recognizes the
hybridization product, resulting from the reaction
between target DNA and a DNA probe. The final
product is revealed by means of a colorimetric
reaction. The DEIA increases the sensitivity of a
previous PCR by including enzymatic reactions.
The hybridization between specific probe and PCR-
amplified target DNA, as well as the formation of
target DNA/probe hybrids and anti-dsDNA antibody
complex, also enhances the specificity.
4. Single-strand conformation polymorphism (PCR-
SSCP): SSCP generally is used as a microbe typing
and mutation detection method. It can also be used
for the purposes of microbe identification. After
PCR products are denatured to two single-stranded
DNAs, the physical conformational changes in single-
stranded DNA are based on the physiochemical
properties of the nucleotide sequence. Conventionally,
the variations in the physical conformation are
detected with non-denaturing polyacrylamide gel
electrophoresis and stained with silver. Also, the
result can be detected by using fluorescence-
labelled primers and analysed on an automated DNA
5. Restriction endonuclease digestions (PCR-RFLP):
After restriction endonuclease digestions, the
amplified DNA fragments are cut to different small
fragments according to their DNA sequences.
The resulting fragments can be separated by gel
electrophoresis, and/or then transferred to a nylon
membrane. RFLP usually is used as a microbe typing
and epidemiological investigative method.
Reverse transcription-polymerase chain reaction
RT-PCR is the technique of synthesis of cDNA from RNA
by reverse transcription (RT) firstly, which is then followed
with amplification of a specific cDNA by PCR. This is the
most useful and sensitive technique for mRNA detection
and quantitation that is currently available. RT-PCR is
mostly used to detect viruses and the viability of microbial
cells through examination of microbial mRNA.
In 1993, Higuchi first described a simple, quantitative
assay for any amplifiable DNA sequence. This method
is based on using fluorescent labelled probes to detect,
confirm, and quantify the PCR products as they are being
generated in real time. In recent years, some commercial
automated real-time PCR systems have been available
(LightCycler & TaqMan). In these systems, such as the
LightCyclerTM and the SmartCycler®, these systems
perform the real-time fluorescence monitoring by using
fluorescent dyes such as SYBR-Green I, which binds
non-specifically to double-stranded DNA generated during
the PCR amplification. Others, such as the TaqMan, use
florescent probes that bind specifically to amplification
target sequences. At present, some broad range primers
and probes targeting the 16S rRNA gene have been
developed in these systems to detect and identify bacteria
universally. The real-time PCR systems not only reduce
the detection time (results can be ready in less than on
hour), but also can reduce contamination risks because
amplification and detection occur within a closed system.
In 1977, two different methods for sequencing DNA were
developed, namely, the chain termination method and
the chemical degradation method. Both methods were
equally popular to begin with, but, the chain termination
method soon become more popular and this method is
more commonly used today. This method is based on the
principle that single-stranded DNA molecules that differ in
length by just a single nucleotide can be separated from
one another using polyacrylamide gel electrophoresis.
The fixed laser beam excites the fluorescently labelled
DNA bands and the light emitted is detected by sensitive
photodetectors. DNA sequence data is the most accurate
and definitive way to identify microbes because the
microbes may be identified by base pair to base pair of
the nucleic acid. The DNA sequences of the variable
regions form the basis of phylogenetic classification of
microbes. By sequencing broad range PCR products,
it is possible to detect DNA from almost any bacterial
species. After comparing the resulting sequences with
known sequences in GenBank or other databases, the
identity of the unknown bacteria can be revealed. Since
the 1990s, 16S rDNA sequencing has become a powerful
tool, which is used more and more in microbial detection
and identification algorithms, especially for unusual,
non-culturable, fastidious and slow growing pathogens,
or after antibiotics that have been administered to the
patient. Such a technique as this is becoming a routine
method of detection and identification of bacteria, thus
overall helping to combat infectious diseases.
Unlike diagnostic virology of which targets DNA as well
as RNA, traditionally the majority of diagnostic assays
in medical bacteriology have been based around the
amplification of DNA in a target gene, as opposed to
mRNA or other nucleic acid signal. This may be performed
as DNA is an extremely stable molecule (Farkas et al.,
1996), as opposed to mRNA which has a short half-life
(Carpousis, 2002; Steege, 2000). Generally molecular
diagnostics in clinical bacteriology is not concerned
with regard to the viable status of a organism being
detected, but is concerned with the qualitative detection
of an organism in a symptomatic patient with the relative
clinical presentation, e.g., the detection of meningococcal
DNA in the cerebral spinal fluid of a paediatric patient with
suspected meningitis. The scenario is completely different
where medical bacteriology interfaces with food/public
health microbiology, where simple qualitative detection of
DNA from pathogenic foodborne bacteria is insufficient or
can indeed be misleading and where other more defined
molecular viability assays are required, e.g. the molecular
detection of viable versus non-viable Salmonella sp. in
a sample of dried milk powder suspected of causing
food-poisoning. In this case, it is insufficient to simply
detect the presence/absence of Salmonella DNA from
an extract of the milk powder as a false positive may be
detected under these circumstances, but which in reality
represents archival DNA from dead cells killed during the
26 Millar et al.
Table 1. Applications of 16S rDNA PCR to identify causal agents of bacterial infection.
Infection Clinical specimen
Bacterial endophthalmitis Vitreous fluid (VF) and aqueous humour (AH) specimensTherese et al. (1998)
Bloodborne sepsisBlood-EDTA Xu et al. (2003)
Chronic prosthetic hip infectionTunney et al. (1999)
Detection of tick infecting bacteriaSchabereiter-Gurtner et al. (2003)
Woo et al. (2003)
Moore et al. (2001b); Goldenberger (1997)
Millar et al. (2001)
Hryniewiecki et al. (2002)
Mueller et al. (1999)
Endodontic infectionsSiqueira et al. (2001)
Febrile episodes in leukaemic patientsLey et al. (1998)
Helicobacter sp. osteomyelitis in an im-
Biopsy of bone lesionHarris et al. (2002)
Intra-amniotic infectionAmniotic fluidJalava et al. (1996)
Intra-ocular infection Carroll et al. (2000)
Maxillary sinus samples from ICU patientsWestergren et al. (2003)
Saravolatz et al. (2003)
Xu et al. (2003)
Nasal polyps. Chronic sinusitis Bucholtz et al. (2002)
Peritonitis CAPD fluid from Culture-negative peritonitisBailey et al. (2002)
Rat bite feverBlister fluidBerger et al. (2001)
Reactive arthritis Synovial fluid/tissueCuchacovich et al. (2002)
Septic arthritisVan der Heijden et al. (1999)
Wound infection Wound tissue from venous leg ulcerHill et al. (2003)
drying process. Therefore it is important to give careful
consideration to what one wishes to achieve from a
molecular assay and thus careful emphasis must be
placed on the target gene locus.
Universal gene targets
Where there is no indication regarding the identity of a
bacterial organism, employment of amplification of DNA
encoding ribosomal RNA genes in conjunction with DNA
sequencing of the amplicon has proven to be valuable
(Patel, 2001; Kolbert and Persing, 1999). In bacteria, there
are three genes which make up the rRNA functionality, i.e.
5S, 16S and 23S rRNA. The 16S rRNA gene has historically
been most commonly employed for identification
purposes (see Table 1), due to it being highly conserved
and having a moderate copy number depending on the
genus. 16S rRNA genes are found in all bacteria and
accumulate mutations at a slow, constant rate over time,
hence they may be used as “molecular clocks” (Woese,
1987). Highly variable portions of the 16S rRNA sequence
provide unique signatures to any bacterium and useful
information about relationships between them. Since
16S rRNA molecules have crucial structural constraints,
certain conserved regions of sequence are found in all
known bacteria. “Broad-range” PCR primers may then
be designed to recognize these conserved bacterial 16S
rRNA gene sequences and used to amplify intervening,
variable or diagnostic regions, without the need to know
any prior sequence or phylogenetic information about the
unknown bacterial isolate.
More recently, employment of the 16S-23S rRNA
intergenic spacer region has become popular due to its
high copy number and more importantly its high sequence
variability (Gurtler and Stanisich 1996; Shang et al., 2003).
Primers are directed to highly conserved regions of the 16S
and 23S rRNA genes and these may either be universal
or specific, targeting a specific genus e.g., Bartonella spp.
(Houpikian and Raoult, 2001), Chlamydia spp. (Madico et
al., 2000), Tropheryma whipplei (Geissdorfer et al., 2001),
Mycobacterium spp. (Roth et al., 2000) and Salmonella
spp. (Bakshi et al., 2002).
Recently, there have been several reports of
employment of the large subunit (23S rRNA) which
have been used to identify bacterial species. Anthony
et al. (Anthony et al. 2000) reported that the 23S rRNA
locus shows more variation between species of medical
importance than the 16S rRNA locus. In their study, these
workers designed universal 23S rRNA primers which
allowed them to detect bacterial agents from 158 positive
blood cultures which were identified using a hybridization
assay with specific oligonucleotide probes. These workers
concluded that the accuracy, range and discrimatory power
of their assay could be continually extended by adding
further oligonuclotides to their panel without significantly
increasing complexity and cost. Furthermore, the 23S
rRNA gene locus has been used specifically in order to
detect Stenotrophomonas maltophila from patients with
cystic fibrosis (Whitby et al., 2000).
Molecular Diagnostics of Medically Important Bacterial Infections 27
Table 2. Commonly employed sequence alignment software tools used in conjunction with nucleotide sequence databases.
Sequence identification tools Nucleotide sequence databases
BLASTn (Basic Local Alignment Sequence Tool)
Ribosomal database project (www.cme.meu.edu/RDP/html/index.html)
MicroSeq (Commercial) (www.appliedbiosystems.com)
SmartGene IDNA (Commercial) (www.smartgene.ch)
Overall, employment of 16S-23S rRNA and 23S rRNA
assays have not been as widely used as those targeting
the 16S rRNA gene, probably due to there being relatively
limited sequence information available for these gene loci
in comparison to the 16S rRNA gene, which has been
traditionally benefited by there being a formal requirement
to describe this rRNA gene in relation to phylogenetic,
taxonomic and population genetic studies. Coupled with
this, presently the only universal bacterial sequence-
based identification scheme available commercially is
based on the 16S rRNA gene i.e. the MicroSeq 500 16S
ribosomal DNA (rDNA) bacterial sequencing kit (Applied
Biosystems, Foster City, CA) (Patel et al., 2000).
rRNA gene loci require the use of software to allow the
identity of the organism to be made. BLASTn and FASTA
software tools are commonly employed to make such
comparisons between the query sequence and those
deposited in global sequence databases, as outlined in
Table 2. Interpretative criteria should be used in order
to ascertain the identification of the unknown sequence
against its most closely related neighbour (Goldenberger
et al., 1997).
identification methods employing
Heat shock proteins
Although 16S rRNA may be employed successfully
to identify many bacterial species, there are regions
within some major genera, in which 16S rRNA gene
sequences are not found to be discriminative enough
for the identification of certain species, for example,
Burkholderia cenocepacia and B. multivorans. In such
circumstances, sequences of essential genes other than
the 16S rRNA, such as the heat shock proteins (HSP)
(HSP60, HSP65, groEL, groER, etc.), have been shown
to be useful (Goh et al., 1996; Woo et al., 2002). The heat
shock response is an important homeostatic mechanism
that enables cells to survive a variety of environmental
stresses. A set of heat shock proteins also known as
chaperonins are induced when cells are exposed to higher
temperatures. This phenomenon has been observed in all
organisms, from bacteria and fungi to plants and animals.
The chaperonins are a well-characterized, subgroup
of molecular chaperones, which includes the GroE
subclass. Heat shock proteins appear to be constituents
of the cellular machinery of protein folding, degradation
and repair (Feltham and Gierasch 2000). This bacterial
molecular chaperone plays an important role in normal
growth by mediating the folding and/or assembly of
different polypeptides, as well as the transport of some
secretory proteins across membranes. For the successful
reactivation and assembly of some proteins, groEL
requires the presence of another heat shock protein,
groES and the general properties of the heat shock
response in many bacteria have been characterized.
Other “universal” gene loci may also be targeted
including the recA locus (Matsui et al., 2001) and the cold
shock proteins (Francis and Stewart 1997). However, the
major disadvantage of employment of these targets is
that there is relatively limited sequence data available for
comparison of a query sequence against their respective
gene sequence databases. For this reason, these targets
are not commonly employed or may be confined to
identification purposes within a well-defined population,
e.g. the Burkholderia cepacia complex of organisms
(Moore et al., 2001a).
Molecular identification of bacteria can utilise specific
gene targets, however in order to do this, prior knowledge
of the sequence is required, so that a specific assay
may be developed. The advantage of using specific
oligonucleotide primers is that they should confer a
higher degree of specificity than employing universal or
broad-range primers. Examples of specific targets that
have been employed include the use of the hippuricase
gene for the differentiation of hippurate-hydrolysing
campylobacters (Camp. jejuni) from non-hydrolysing
campylobacters (Slater and Owen 1997) or the use of
ctrA gene for meningococci in the laboratory diagnosis
of meningococcal meningitis (Guiver et al., 2000). The
specific target does not necessarily have to be associated
with a PCR assay, but may be used in combination
with several other nucleic acid amplification/analysis
techniques (see Table 3).
Presently, there are several hundred specific assays
available for the identification of a diverse variety of
bacteria and too numerous to detail in this section.
However, the end user of a given assay should be aware
that each assay is potentially troubled with several pitfalls
of poor assay design, which may lead to poor specificity
and/or sensitivity. Therefore, before any assay is adopted
into routine diagnostic service, the published method must
firstly be empirically optimised in the end user’s laboratory
and that the user has an appreciation of the strengths and
28 Millar et al.
Table 3. Specimen diversity and molecular diagnostic work flow of medically important bacterial infections.
Amniotic fluid Jalava et al. (1996)
Arterio-embolic tissue Mueller et al. (1999)
Ascitic fluid Such et al. (2002)
Atheroma Apfalter et al. (2002)
Blood culture Millar et al. (2001)
Blood-EDTA Xu et al. (2003)
Bone Harris et al. (2002)
Bone marrow Gamboa et al. (1997)
Breast milk Schmidt et al. (1995)
Bronchioelar lavage (BAL) Ersch et al. (2000)
Cerebral spinal fluid (CSF) Saravolatz et al. (2003)
Cervical specimen/tissue Lehmann et al. (1999)
Culture isolate (pure in vitro) Millar et al. (2001)
Feces Kabir (2001)
Fixed tissue sample Wu et al. (2002)
Heart valve Gauduchon et al. (2003)
Lymph node tissue Yamada et al. (2002)
Middle ear fluid Jero et al. (1999)
Nasal polyps/sinus/lavage Bucholtz et al. (2002)
Paraffin-embedded tissue Yamada et al. (2002)
Pus (wound and blister) Berger et al. (2001); Kox et al. (1995)
Plasma Klaschik et al. (2002)
Pleural effusion/fluid Yanagihara et al. (2002)
Prosthetic device Tunney et al. (1999)
Saliva Sakamoto et al. (2002)
Semen/sperm Gdoura et al. (2001)
Serum Such et al. (2002)
Skin Torres et al. (2003); Enzensberger et al. (2002)
Sputum McDowell et al. (2001)
Swabs Torres et al. (2003)
Synovial fluid Vander Heijden et al. (1999)
Tissue (wound) Hill et al. (2003)
Urine Mahony et al. (1997)
Vaginal fluid Obata-Yasuoka et al. (2002)
Vitreous humour Sharma et al. (2002)
Nucleic acid extraction
Alkali/heat lysis Millar et al. (2000)
Automated DNA extraction (e.g. MagNAPure)Raggam et al. (2002)
Boil Clarke et al. (2003)
Centrifugation/Chelex-100/boil Schmidt et al. (1995)
Commercial Kits (e.g., Qiagen/Roche) Millar et al. (2000); Clarke et al. (2003)
“In house methods” Millar et al. (2000)
Phenol/chloroform Moore et al. (2002)
Silica capture/guanidine hydrochloride treatment/proteinase K/lysozyme Millar et al. (2000)
weaknesses of the assay, so that interpretation of the end
result is easier to make.
Antibiotic resistance markers
Recently, antibiotic resistance in bacterial pathogens
has become an important topic both nationally and
internationally. Some scientists are forecasting the
emergence of the “post antibiotic era”, where it will
be difficult to control common infections, due to the
emergence of high level resistance in several medically
important bacterial pathogens. Consequently, there has
been great interest in being able to detect antibiotic
Molecular Diagnostics of Medically Important Bacterial Infections 29
Table 3. Continued.
Nucleic acid amplification/analysis
Block-based PCR Tang et al. (1997)
– single round Millar et al. (2001)
– semi-nested Moore et al. (2002)
– nested Hinrikson et al. (2000)
– multiplex Weaver and Rowe (1997)
Branched DNA signal amplification Tang et al. (1997); Zheng et al. (1999)
DNA-hybridization/probe assay Tang et al. (1997)
In situ PCR/RT-PCR Jin and Lloyd (1997)
Ligase chain reaction (LCR) Blocker et al. (2002); Bachmann et al. (2002); Wang and Tay (2002); Gilpin et al.
(2002); Castriciano et al. (2002); Rajo et al. (2002); Friese et al. (2001)
Microarrays Anthony et al. (2001)
Nucleic acid sequence-based amplification (NASBA)Cook (2003); Cook et al. (2002)
PCR-target capture/hybrid capture Maibach et al. (2002)
Q β replicase system
Tang et al. (1997); An et al. (1995)
Real-time PCR Klaschik et al. (2002)
-Light cycler O’Mahony et al. (2002)
-Taqman Ellerbrook et al. (2002)
Reverse transcriptase PCR (RT-PCR) Tang et al. (1997)
Strand displacement amplification (SDA) Ge et al. (2002)
Transcription mediated amplification (TMA) Hill (2001)
Characterization/Identification of amplicon
Automated sequence analysis Patel (2001)
DNA-DNA hybridization Siqueira et al. (2001)
Enzyme immunoassay (PCR-EIA) Moreno et al. (2003)
Restriction enzyme analysis (PCR-REA) Brown and Levett (1997)
Restriction fragment length polymorphism (RFLP) McDowell et al. (2001)
Single-strand conformational polymorphism (SSCP) Kerr and Curran (1996); Hein et al. (2003)
Amplified fragment length polymorphism (AFLP) Moreno et al. (2002)
Arbitrary-primed – PCR (AP-PCR) Dabrowski et al. (2003)
Multilocus sequence typing (MLST) Enright and Spratt (1999)
Pulsed field gel electrophoresis (PFGE) Wu and Della-Latta (2002)
Random amplification of polymorphic DNA-(RAPD) Power (1996)
- BOX PCR Gillespie (1999)
- ERIC PCR Marty (1997)
- rep PCR Baldy-Chudzik (2001)
Ribotyping Moore et al. (2002a)
Single-strand conformational polymorphism (SSCP)Kerr and Curran (1996); Hein et al. (2003)
resistance molecular, particularly when the causal
agent is fastidious or non-culturable. Fluit et al., (2001)
have recently published an authoritative review on the
molecular detection of antibiotic resistance. Currently,
most hospitals are concerned with the occurrence of
methicillin resistance Staphylococcus aureus (MRSA)
and glycopeptide resistant enterococci (GRE) on their
wards, particularly surgical wards. Several workers have
published molecular methods to detect MRSA through
employment of a simple PCR assay, targeting the mecA
gene locus (Kobayashi et al., 1994; Towner et al., 1998).
The employment of mecA PCR to screen for carrier status
of patients in the intensive care unit (ICU) is a useful tool to
allow infection control teams to segregate positive MRSA
patients from non-colonised patients, thereby minimising
the opportunity for nosocomial spread amongst ICU
patients. Application of such an assay also demonstrates
that for this screening policy to be effective, the host
laboratory should be in a position to perform the assays
locally and not rely on sending cultures to a Reference
Genomovar analysis of Burkholderia cepacia in
Infection with the Burkholderia cepacia complex (BCC) is
an important cause of increased morbidity and reduced
30 Millar et al.
survival in patients with cystic fibrosis (CF) (Høiby
1991). Certain members of the BCC are transmissible
and epidemics have been described in a number of CF
centres (Doring et al., 1996). A number of factors such
as the presence of the B. cepacia epidemic strain marker
(BCESM) (Mahenthiralingam et al., 1997) and the cable
pilus gene (Sajjan et al., 1995) have been identified as
markers of transmissibility. Most units in the UK now
segregate patients with this infection from all other CF
patients and infection control guidelines have been
recently published by the UK CF Trust (Anon., 1999) to
help reduce the potential for cross infection with these
organisms. Nine genomovars of the BCC have now
been formally described and initial studies indicate that
genomovar II has less clinical impact than genomovar
III (De Soyza et al., 2000). Presently B. cenocepacia
(formerly genomovar III) followed by Burkholderia
multivorans (formerly B. cepacia genomovar II) make up
the majority of patients infected with BCC. Some centres
now segregate patients with different genomovar types
eg., genomovar III patients from other B. cepacia complex
infected patients (e.g. Belfast and Vancouver) to reduce
the risk of cross infection between patients with BCC.
Presently some centres believe that complete segregation
of all the genomovar types is the best policy, whilst others
feel that it is only important to segregate genomovar III
from the other BCC types. Early accurate identification
of BCC is of critical importance so the patients can
be segregated and therefore reduce the potential for
further epidemics. BCC organisms present the clinical
microbiologist with a diagnostic dilemma, in that there
are extremely few and in some cases no phenotypic
biochemical or growth-related characterization tests that
reliably distinguish between these organisms. To this end,
there have been a variety of different molecular-based
characterization tests to differentiate the genomovars
(Segonds et al., 1999; LiPuma, 111999). Recently, the
recA gene has been shown to aid in the differentiation
of the genomovars (Mahenthiralingam 2000). RecA is a
multifunctional and ubiquitous protein involved in general
genetic recombination events and in DNA repair. It
regulates the synthesis and activity of DNA repair (SOS
induction) and catalyses homologous recombination and
mutagenesis. As this locus may contribute significantly
to the overall genomic plasticity of the BCC, this locus
is potentially a strong candidate for the adoption of rapid
molecular-based assays in the laboratory. Furthermore,
molecular methods may aid CF centres in helping
determine the status of CF patients with regard to whether
they are chronically, transiently or not colonised with this
Molecular diagnostic procedures
Most molecular assays rely on three basic components,
including nucleic acid extraction, amplification/analysis
and detection of an amplified product, as outlined in
Table 3. Most molecular assays allow for a wide variety
of permutations and combinations of methods, depending
on what is trying to be achieved. For example, almost all
clinical specimen types have been extracted by a variety
of DNA extraction protocols. Choice of nucleic acid
amplification/analysis may be varied, however presently,
application of “real-time” platforms including the Roche
LightCycler and the ABI Taqman 7700 systems are popular,
as these not only allow detection in a relatively short
time in a gel-less system, but also for the quantification
of copy numbers. There are several factors which help
determine which type of assay to employ (see Table 4). If
speed is an important factor of the assay, e.g. detection of
meningococcal DNA in CSF from children with suspected
meningitis, employment of the real-time assays should
be adopted. Where numerous targets are important, the
multiplex PCR format should be employed.
Laboratory management of molecular assays
All molecular assays, due to their high sensitivity,
are prone to contamination problems, which has the
potential to lead to false-positives, thus diminishing the
effectiveness of such testing regimes and thus strict
contamination controls need to be established to avoid
the occurrence of such problems. The use of broad-range
PCR is particularly susceptible to contamination problems
(Millar et al., 2002) and it has also been argued that such
problems are indeed not unique to broad-range PCR, but
that can be equally applied to specific PCR (Bastien et
al., 2003). From clinical specimen reception to molecular
analysis, there are numerous sources of contamination
risk and for each molecular assay these should be
identified and appropriate control measures identified
and established to minimize each risk (see Table 5 for
working example). To ensure the minimum contamination
risk, including PCR amplicon carry-over, a key element
is successful workflow through geographically separated
areas. It is recognized that many diagnostic facilities in
hospital laboratories have limited work space or have
poor quality work space that makes separation of pre-
and post-PCR areas difficult to achieve. However, it is
important that adequate space is allocated to ensure
compliance and to conform to CPA Accredittion standards
for molecular diagnosis (Anon., 1999).
Successful employment of PCR in the detection of
causal agents of infectious disease is critically dependent
on both the quantity and quality of controls associated with
the assay, to avoid the occurrence of both false-positive
and false-negative results. Table 6 lists the reasons for
false-positive and false-negative findings. It should be
noted that for each PCR-based test, sensitivity should
be evaluated for each specimen type, prior to routine
implementation. It is vital that several negative and positive
controls are set up during each diagnostic run. Negative
and positive controls should include (a) DNA extraction
control, (b) PCR set up control and (c) PCR amplification
control. For DNA extraction purposes, the positive control
should include clinical specimen artificially spiked with
organism, e.g. blood culture spiked with E. coli. For
clinical tissue specimens where a true positive specimen
may be difficult to mimic, internal positive controls may be
employed, such as following DNA extraction, amplification
of the β-globin gene, as previously described (Millar et al.,
2001). With respect to PCR controls, the positive control
should be bacterial DNA extracted from a pure culture.
Ideally, the positive control should include two components,
namely: (i) a specimen generating a weak signal, due to
low copy numbers of target and (ii) a specimen generating
Molecular Diagnostics of Medically Important Bacterial Infections 31
Table 4. Criteria used in the determination of the most appropriate molecular method for use in bacterial infections.
Criterion Appropriate molecular method of choiceComment/example
Time/speed to detectionReal-time applications (LightCycler/TaqMan 7700 system) Detection of meningococcal DNA from clinical
specimens in children with suspected meningitis
(Guiver et al. 2000)
Quantification LightCycler/TaqMan 7700 system/NASBA Determination of effect of antibiotic intervention
Multiple targets Multiplex PCR/microarrays/hybridization probe assayDetermination of multiple respiratory pathogen
ViabilityNASBA/RT-PCR Determination of viable pathogens in foodstuffs
or detection of viable but non-cultural (VNC)
organisms (Cook, 2003)
Commercial availabilityMicroSeq (ABI Ltd.), LCR (Abbott) Identification by comparison of query organ-
ism with high quality database (4). Detection of
Chlamydia in genital specimens (Bachmann et
al., 2002, Castriciano et al., 2002, Friese et al.,
Throughput High – Real-time PCR
Low – Block-based PCR
Automated DNA extraction followed by real-time
Table 5. Potential sources of DNA contamination and its remediation measures in molecular diagnostic analyses.
Specimen collection Nucleic acid extractionPCR analysis
Location: hospital ward
Personnel: nurse, junior doctor
Method: Commercial Kit
Additional reagents: lysozyme, water,
Location: class II biological safety cabinet
Reagents: water, buffer, MgCl2, dNTPs, Taq,
Location: PCR cabinet in different location to
nucleic acid extraction and post PCR
1. Commensal flora on patient’s
2. Commensal flora on staff’s
3. Archival DNA in specimen vial
4. Archival DNA in EDTA solution
5. Inappropriate collection of
specimen by personnel
1. DNA from kit reagents
2. DNA from additional reagents
4. Class II biological safety cabinet
6. Equipment e.g., centrifuge, heating
7. Reaction vials (1.5ml)
1. DNA from reagents
3. PCR cabinet
5. Equipment e.g., vortex
6. Reaction vials (1.5ml) and PCR tubes
1. Iodine scrub of puncture site
2. Wearing of sterile gloves and
3/4. Employment of laboratory
prepared and quality controlled
DNA-free blood-EDTA vials
5. Education of personnel
1/2. All reagent purchased should be of
All reagents should be screened prior to
Contaminating DNA should be monitored
using negative DNA extraction controls
3. A dedicated pre-PCR room should be
used, ideally under positive pressure
Unidirectional work flow
4. Cabinet should be cleaned thoroughly
and UV irradiated for a minimum period of
2h prior to use
Cabinet should be serviced regularly
5. Dedicated pipettes should be used and
should not be removed from the class II
Plugged sterile tips should be employed
Pipettes should be cleaned and the barrels
UV irradiated for a minimum of 2h prior to
6. Equipment should be dedicated to
extraction purposes only
7. Reaction vials should be pre-autoclaved
8. Education of personnel.
Use of sterile gloves and dedicated labora-
tory clothing for DNA extraction purposes
9. Avoidance of re-usable laboratory
1. All reagents purchased should be of molecular
All reagents should be screened prior to use.
PCR master mix minus the primers and template
DNA should be UV irradiated for 15 min prior to
Contaminating DNA should be monitored using
negative PCR setup controls
2. A dedicated pre-PCR room should be used,
ideally under positive pressure and separate from
DNA extraction procedures.
Unidirectional work flow
3. Cabinet should be cleaned thoroughly and UV
irradiated for a minimum period of 2h prior to use.
Cabinet should be serviced regularly
4. Dedicated pipettes should be used and should
not be removed from the PCR setup cabinet.
Plugged sterile tips should be employed through-
out. Pipettes should be cleaned and the barrels
UV irradiated for a minimum of 2h prior to use
5. Equipment should be dedicated to PCR setup
6. Reaction vials and PCR tubes should be pre-
autoclaved and DNA-free.
7. Education of personnel. Use of sterile gloves
and dedicated laboratory clothing for PCR setup
8. Avoidance of re-usable laboratory glassware
32 Millar et al.
Table 6. Most common factors contributing towards false positive and false negative results in molecular diagnostic assays.
Reasons for false-positive results Reasons for false-negative results
Carry over contamination (amplicons) from previously amplified productsInhibition of PCR reaction
Presence of exogenous target DNA in reagents, water, kits, sterile blood
Inadvertent loss of template nucleic acid target due to poor extraction,
handling and storage protocols
Poor primer design (non-specificity) Digestion of nucleic acid template with endogenous DNAses and RNAses
Inadequate amplification conditionsPoor primer design (non-conserved regions at primer site(s) in variants)
Contamination from laboratory personnelPoor intrinsic sensitivity of nucleic acid amplification/analysis detection
Poor sensitivity of nucleic acid amplification/analysis reaction
Inadequate amplification conditions
a strong signal, due to high copy numbers of target. By
employing these controls especially the negative controls,
it is easier to identify the point of contamination within the
diagnostic assay e.g., DNA extraction contamination free
but contaminated at PCR set up stage. Positive controls
are also important, particularly those included in the DNA
extraction procedures, as they serve to identify possible
inhibition of the PCR reaction, due to inhibitory agents in
the biological specimen, which co-elute with extracted
DNA, e.g. sodium polyanetholesulfonate in blood culture
material (Millar et al., 2001). For a comprehensive review
on PCR inhibition with respect to biological specimens,
see Wilson (Wilson 1997).
Standardization and harmonization
Presently there are numerous molecular methodologies
for the identification and genotyping of both culturable
and non-culturable bacterial causal agents of infection.
Although various commercial approaches have been
described, such as the MicroSeq PCR-DNA sequencing
identification system, the majority of methods used in
clinical microbiology laboratories are “in-house”. To date,
there has been little or no attempts made to standardize
and harmonize bacterial detection protocols between
laboratories at a local, national and international level,
although several European centres have attempted to
examine these for specific organisms (Struelens and
others 1996; Deplano et al., 2000; Dijkshoorn et al.,
2001; Fry et al., 1999; Grundmann et al., 1997; Van
Belkum et al., 1998). The disadvantage of not employing
such standardized methods may lead to anomalies in
the epidemiologically, which may yield bias results and
hence corrupt the epidemiological reporting of a variety
of infectious disease states. Most diagnostic laboratories
employ methods, which are not completely consistent with
their peer laboratories, e.g. employment of a published
method with variations in the empirical optimization
and/or with different reagent suppliers. Presently, there
are limited studies detailing the effect of such variation
on qualitative reporting of results and hence this area
requires urgent attention. More recently, various focussed
attempts have been made to try to standardize bacterial
subtyping techniques, through the actions of PulseNet
(http://www.cdc.gov/pulsenet/), primarily for bacterial
foodborne disease surveillance in the US and the ESF
Network for Exchange of Microbial Typing Information
European Network (ENEMTI) (http://lists.nottingham.
ac.uk/mailman/listinfo/enemti). ENEMTI is a network
of European laboratories that aims to standardize
methods and data exchange protocols for internet-based
comparison of microbial fingerprinting data. This project
aims to develop an internet-based database system for
DNA fingerprints that is readily accessible and user-
friendly for microbiologists with only limited computer
expertise. In addition, the European Society for Clinical
Microbiology and Infectious Disease (ESCMID) have a
specific working group, namely the ESCMID Study Group
on Epidemiological Markers (ESGEM), whose objectives
are to (a) critically evaluate microbiological typing systems
and make recommendations for their appropriate use, (b)
promote collaborative research into microbiological typing
systems and to develop standardized methodology for
specific pathogens, (c) provide a forum for the exchange
of ideas and the development of consensus strategies
and (d) work with individuals and companies active in
this research area to foster the development of further
technological advances in microbial typing (http://www.
Appraisal of molecular diagnostics in clinical
Molecular diagnostics have several advantages and
disadvantages for their adoption into routine bacteriology,
as detailed in Table 7. At present, employment of
molecular diagnostics is largely confined to specialized
or reference bacteriology laboratories, due to several
factors. With such an approach, molecular methodologies
including PCR, real-time PCR and PFGE may have
the opportunity to become adopted by several regional
diagnostic laboratories outside of highly specialized
reference facilities, as such methodologies have been
shown to provide valuable real-time information to aid
in outbreak management and identification of non-
culturable or fastidious organisms. To date, the speed in
which these assays have been taken up has been closely
related to the relative skills base within the laboratory,
thus although there is an advantage in all hospital types
utilising such technology i.e. from the district general to
university teaching hospitals, such techniques have not
become common where the skills base does not exist.
Furthermore, there is the added danger that hospital
research facilities, which have been the custodians of
these research techniques to date, become overloaded
with a molecular diagnostic workload, simply because
they have the necessary skills base. In addition, molecular
techniques are perceived to be relatively expensive,
which is the case on a simple head-to-head comparison
(Table 8). The overall value of the quality of the test result,
Molecular Diagnostics of Medically Important Bacterial Infections 33
Table 7, Advantages and disadvantages of the adoption of molecular assays into clinical bacteriology.
Diagnostic criterion AdvantagesDisadvantages
Accuracy of identificationAid in identifying aetiological agents of infections
which are difficult to culture, including:
Slow growing organisms
Category 3 cultures where a designated secure
cell culture laboratory is required
Difficult, specific culture requirements where
limited serological tests exist
identification of causal agent following antibiotic
Problems associated with contaminant organisms, however
these problems may be aided by inclusion of appropriate
DNA extraction and PCR controls (Millar et al. 2002)
The agent identified should be considered with respect to the
patient’s medical and general history
Time to detection
Where specimens are:
a. culture-positive and/or serol-
b. culture-negative and/or serol-
Confirmation of conventional detection result
more rapid detection than conventional culture for
fastidious and cell-dependent organisms
Confirmation of serology result. Detect and high-
light non-specific serological false-positive results
Longer time required for molecular PCR and sequence
analysis than culture & serology for non-fastidious or cell-
Longer time required for molecular PCR and sequence
analysis than serology
c. culture-negative and serology-
Identification of causal agent when all conven-
tional diagnostic assays are negative
Impact on therapyAppropriate antibiotic therapy can be commenced
sooner or modified earlier in the presence of a
molecular identification from a culture-negative/se-
Provision for PCR detection of antimicrobial
resistance gene determinants in culture-negative
Cost-effectiveness Cost-effective particularly with culture-negative/
serology-negative specimens to avoid extended
analysis for several potential pathogens either by
specific culture and serological testing.
Economic and early use of most appropriate cost-
effective antibiotic treatment regimen
Economic and optimised in-patient stay
Not cost effective when conventional culture and serology
give quality and early identification result
Necessary to purchase/lease specialised equipment usually
with costly maintenance contracts
Lack of education in modern molecular based technologies
Medical laboratory scientific officers, clinical scientists and
medical microbiologists all must understand the principles of
molecular based technologies to ensure proper handling of
the specimens and appropriate interpretation and significance
of results (Moore and Millar 2002), hence specific training
must be given
Table 8. Comparison of financial cost of identification of a bacterial culture employing phenotypic and genotypic identification schemes.
Routine/conventional (API) identification Molecular (16S rDNA PCR & sequencing) identification
Consumable item Approx. cost (GBP £) (ex VAT)Consumable item Approx. cost (GBP £) (ex VAT)
1 x API20NE strip3–95 DNA extraction kit 2–50
API diagnostic reagentsa
0–80 PCR reagents (not including primers)0–12 (Taq) + 0–14 (dNTPs)
Specialist reagents (TAE, Tris, EtBr, etc)0–10
PCR primers (forward and reverse)0–10
PCR sub-total costs5–55 x 2 = 11–10
Sequencing primers (Cy-5’ labelled)0–20
Sequencing sub-total costs4–59 x 2 = 9–18
aAssuming one set of reagents are adequate for two kits, bassuming the amplicon is sequenced as part of a ten-set batch.
34 Millar et al.
however, should be taken into account in terms of several
parameters, including time-to-detection and ability to
detect an agent. Although the use of a molecular test is
albeit more expensive, its employment may yield a finding
that could potentially reduce costs significantly further
downstream with patient management.
In conclusion, molecular diagnostic techniques
have a significant role to play in clinical bacteriology,
although their adoption will never replace conventional
methodologies, which continue to be the cornerstone of
modern bacteriological methods. Indeed, such molecular
diagnostic assays may only be implemented in specialized
laboratories to enhance laboratory diagnostic efficiency,
where the use of such assays will be mainly confined to
diagnosis, identification and genotyping, where current
conventional approaches are grossly inadequate. Adoption
of such methods in bacteriology has occurred at a much
slower rate than in clinical virology, where the inadequacies
of conventional virology, has accelerated the adoption of
molecular methods. Integration of molecular approaches
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