ANTIBIOTIC SUSCEPTIBILITY TESTING: A REVIEW ON CURRENT PRACTICES

Abstract

An increasing Antimicrobial resistance (AMR) has resulted in morbidity and mortality from treatment failures and increased health care costs. Appropriate antimicrobial drug use has unquestionable benefit, but physicians and the public frequently use these agents inappropriately hence, it became necessary to perform the antimicrobial susceptibility test as a routine. The aim of antimicrobial susceptibility testing is to determine the lowest concentration of existing or even new antimicrobial agents which inhibits the visible growth of the bacterium being investigated, under certain test conditions. The Disk diffusion, well diffusion, stokes and gradient diffusion methods are manual methods that provide flexibility and possible cost savings. The most commonly used testing methods include broth microdilution method using commercially available 96-well micro dilution panel. Broth dilution, tube dilution and E test provide quantitative results (e.g. Minimal Inhibitory Concentration) whereas other methods provide qualitative results which are categorized as susceptible, intermediate or resistant. Although available testing methods provide accurate detection of common antimicrobial resistance mechanisms, emerging newer mechanisms of resistance certainly attracts researcher for the development of advanced, reproducible, automated and reliable antimicrobial testing methods.

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Uddhav and Sivagurunathan. Int J Pharm 2016; 6(3): 11-17 ISSN 2249-1848
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Review Article CODEN: IJPNL6
ANTIBIOTIC SUSCEPTIBILITY TESTING: A REVIEW ON CURRENT PRACTICES
Uddhav S Bagul*and Sivagurunathan M. Sivakumar
College of Pharmacy, Jazan University, Jazan, Kingdom of Saudi Arabia
*Corresponding author e-mail: usbagul.siop@yahoo.in
Received on: 25-03-2016; Revised on: 07-04-2016; Accepted on: 15-05-2016
ABSTRACT
An increasing Antimicrobial resistance (AMR) has resulted in morbidity and mortality from treatment failures and
increased health care costs. Appropriate antimicrobial drug use has unquestionable benefit, but physicians and the
public frequently use these agents inappropriately hence, it became necessary to perform the antimicrobial
susceptibility test as a routine. The aim of antimicrobial susceptibility testing is to determine the lowest
concentration of existing or even new antimicrobial agents which inhibits the visible growth of the bacterium being
investigated, under certain test conditions. The Disk diffusion, well diffusion, stokes and gradient diffusion methods
are manual methods that provide flexibility and possible cost savings. The most commonly used testing methods
include broth microdilution method using commercially available 96-well micro dilution panel. Broth dilution, tube
dilution and E test provide quantitative results (e.g. Minimal Inhibitory Concentration) whereas other methods
provide qualitative results which are categorized as susceptible, intermediate or resistant. Although available testing
methods provide accurate detection of common antimicrobial resistance mechanisms, emerging newer mechanisms
of resistance certainly attracts researcher for the development of advanced, reproducible, automated and reliable
antimicrobial testing methods.
Keywords: Disk diffusion, zone of inhibition, minimum inhibitory concentration, microdilution, E-test
INTRODUCTION
Antibiotics/antimicrobial agents are the major drugs
of choice of the physician’s desk to treat the
pathogenic infections. It has been observed that some
of the clinicians prescribe the medicine based on the
symptoms instead of performing diagnostic tests.
This prescribing pattern may be one of the reasons
for the development of resistant for the antibiotics [1].
Therefore, Antibiotics susceptibility testing (AST)
plays an important role to check the effectiveness of a
drug against a bacterium and select the best drug that
act against the bacterium. One of the significant roles
of clinical microbiology laboratory is the
performance of antimicrobial susceptibility testing of
various bacterial isolates. The main objectives of the
testing are to find out possible drug resistance in
common pathogenic microorganism and the
susceptibility to drug of choice for a particular
infectious microorganism can be assured.
Mechanism of antimicrobial resistance: There are
number of ways by which microorganisms are
resistant to antimicrobial agents. These includes: 1.
Bacteria produce enzymes which destroy the
antimicrobial agents before it reaches its targets e.g.
Beta lactamase enzyme hydrolyses beta lactam drugs
which develop resistance. 2. Impermeable cell for
antimicrobial drugs e.g. Gram negative bacteria may
become resistant to Beta lactam antibiotics by
developing permeability barrier. 3. Mutation e.g.
Ribosome methylation of ribosomal RNA develop
macrolide resistant. 4. Bacterial efflux pump that
expels antimicrobial drugs from cell before it can
reach its targets. 5. Specific Metabolic pathways in
the bacteria are genetically altered so that
antibacterial agents cannot exert an effect [2, 3].
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Purpose of antimicrobial susceptibility testing
a) A laboratory test which determine that how
effective antibiotic therapy is against a bacterial
infection. b) AST can control the use of antibiotics in
clinical practice. c) AST testing will assist the
clinicians in the choice of drug for the treatment of
infection. d) AST can help the local pattern of
antibiotics prescriptions. e) To reveal the changing
trends in the local isolates.
Antimicrobial susceptibility testing methods:
A. Qualitative Method: This method is used for
testing of isolates from healthy patients with intact
immune defenses, in less serious infections such as
UTI. There are two qualitative methods.
Disk diffusion test: The disk diffusion sensitivity test
also known as Kirby Bauer disk method “Figure 1”,
is a simple and practical which uses antibiotic-
impregnated wafers (disk) to test whether particular
bacteria is susceptible to specific antibiotic or
otherwise [4-6]. The bacterial inoculum (approximately
1-2 X 108 CFU/mL) was uniformly spread using
sterile cotton swab on a sterile Petri dish MH agar.
The antibiotic disks were placed on top of the
previously inoculated Mueller Hinton agar medium
surface with the help of sterile forceps. Each disc
must press down to ensure complete contact with the
agar surface. The plates were incubated for 18–24 h
at 35-37 0C temperature in bacteriological incubator
before an interpretation of the result.
The antibiotic diffuses from the disc into the agar in
decreasing amounts the further it is away from the
disk. If the organisms were killed or inhibited by the
concentration of the antibiotic, there will be no
growth in the immediate area around the disks
represented as zone of growth inhibition. The
diameter of the zone of inhibition is directly
proportional to the sensibility of the isolate and to the
diffusion rate of antibiotics through the agar medium.
A zone of inhibition was measured in millimeters by
either measuring: (A) Radius: Measure half the
distance of the zone and then multiply by 2. This
method was used when part of the zone is not clear or
has grown into another zone. (B) Diameter: Measure
the entire length of the zone and subtract the disk
diameter (Standard disk size 5-6mm). The result of
the test can be interpreted by using the criteria
published by Clinical and Laboratory Standard
Institute (CLSA formerly the National Committee for
the Clinical Laboratory Standard or NCCLS) [7].
The results of the disk diffusion test are “qualitative”
and will be reported out as:
Susceptible: ‘The term “susceptible” represent that
isolates are inhibited by the usually recommended
dosage of an antimicrobial agents. However, this
term doesn’t assure clinical success; in fact predicting
clinical outcome based on susceptibility testing and
the use of drugs shown to be in the susceptible
category is very imprecise. This imprecision is due to
the effect of host responses, site of infection, toxin
production by bacteria that is independent of
antimicrobial susceptibility, the presence of biofilm,
drug pharmacodynamics and other factors.
Intermediate: ‘The “intermediate” category includes
isolates with antimicrobial MICs that approach
usually attainable blood and tissue levels and for
which response rates may be lower than for
susceptible isolates. The intermediate category
implies clinical efficacy in body sites where the drugs
are physiologically concentrated (e.g. quinolones and
beta-lactams in urine) or when a higher than normal
dosage of a drug can be used (e.g. beta lactams).
Resistant: ‘The category indicates that isolates are
not inhibited by the usually achievable concentrations
of the antibiotics with normal dosage schedules,
which demonstrate an existence of the specific
microbial resistance mechanisms (e.g. beta-
lactamases).
The merits of the disk diffusion methods are
simplicity in test, most economic, flexibility in disk
selection, and the result can be easily interpreted by
clinicians. However, the demerits include manual
test, lack of automation and all fastidious or slow
growing bacteria cannot be accurately tested by this
method. The limitation of this testing show that the
microbiologist and clinician both should not forget
that the response therapy in vivo may not always
reflects the result of testing the sensitivity of patient’s
pathogen in vitro. Rakesh Kumar exploited this
method to study antimicrobial sensitivity pattern of
Escherichia coli from urine samples of UTI patients
[8].
Well diffusion method: In this agar well diffusion
method, a suitable agar medium was prepared, once
the agar is solidified the medium was inoculated and
swabbed with bacterial suspension of approximately
1-2 X 108 CFU/mL using cotton swab. The wells
were prepared by punching with a six millimeters
diameter standard sterile cork borer made up of
stainless still “Figure 2”. These wells were filled up
with 25 – 50 μL of the antimicrobial solution/s. to be
tested. Well diffusion test has been used for
susceptibility testing of antifungals like fluconazole,
itraconazole [9, 10]. The plates were incubated at 35 ±

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2°C for 18 – 24 h. The antimicrobial activity is
calculated in millimeter by using the expression: ZOI
= Total Diameter of growth inhibited zone minus
diameter of the well, where, ZOI is Zone of
inhibition.
The factors which may affect the result of AST
included Density of an inoculum, Disk application
time, Temperature of incubation, Potency of drug,
inappropriate storage conditions, pH the agar
medium, Moisture on the surface of the medium and
effects of Thymidine or Thymine containing agar
medium [1].
Stokes method: Stokes Disc Diffusion Technique
varies from Kirby Bauer disc diffusion in the use of
both control and test strain on a same plate. Stokes
disc diffusion technique is not as highly standardized
as the Kirby-Bauer technique and is used in
laboratories particularly when the exact amount of
antimicrobial in a disc cannot be guaranteed due to
difficulties in obtaining discs and storing them
correctly or when the other conditions required for
the Kirby-Bauer technique cannot be met.
Comparative disc diffusion techniques based on
Stokes method is still in wide use in majority of
laboratories in UK, to determine antibiotic
susceptibility. The stokes’ method allows each
individual isolate to be compared with a sensitive
control of the same or similar species which is
subjected to the same technical conditions of
medium, incubation time, atmosphere, temperature
and disc content. As control and test organisms are
adjacent on the same plate the difference between
respective zone sizes can be measured directly.
In this method a control sensitive bacterial culture
were inoculated partly on the surface of Mueller
Hinton agar plate and a bacterial suspension to be
tested was inoculated on the remainder part of the
agar plate. The antibiotic disks were placed exactly at
the interface “Figure 3”. The plates were incubated at
35 -37 0C temperature for 18 – 24 h. The control and
the test results for zone of inhibition were compared.
The use of a sensitive control shows that the
antibiotic was active and if the growth was observed
on test area it may safely be assumed that the test
organism was resistant to that drug. The bacterial
culture was susceptible to drug “x” but resistant to
drug “y”. However, the disc containing drug “y”
represent that an active antibiotic is present in the
disk as shown by the zone of inhibition it causes in
the control bacterium [11].
The Advantages of Stokes method includes: the
control strain and test strain can be checked on the
same plate. More reliable for the quality testing of
discs. The effect of variation of environmental
condition like temperature, time affect both
simultaneously thus minimizing error. Errors due to
using too heavy or light inoculums will be detected.
B. Quantitative method: This method is applied in the
treatment of severe infections such as endocarditis or
osteomyelitis. The principle of this method is based
on the dilution and diffusion and dilution together.
Dilution susceptibility testing methods are used to
determine the minimal concentration of antimicrobial
to inhibit or kill the microorganism. This can be
achieved by dilution of antimicrobial agents in either
agar or broth media. The aim of the broth and agar
dilution methods is to determine the lowest
concentration of the antimicrobial that inhibits the
visible growth of the bacterium being tested (MIC,
usually expressed in μg/ml or mg/liter). However, the
MIC does not always represent an absolute value.
The ‘true’ MIC is a point between the lowest test
concentration that inhibits the growth of the
bacterium and the next lower test concentration.
Hence, MIC determinations performed using a
dilution series may be considered to have an inherent
variation of one dilution.
Tube or Macro broth dilution test: In the broth
dilution method antibiotic solutions are prepared by
two fold dilutions (1, 2, 4, 8, 16, 32 and 64 μg/mL) in
the liquid growth medium dispensed in the test tubes.
The standardised bacterial suspension of 1-5 X 105
CFU/mL was inoculated in the antibiotics containing
tubes “Figure 4”. These tubes were incubated for 16–
20 h at 35–37 oC temperature and observed for
visible bacterial growth as judged by turbidity.
Minimum inhibitory concentration (MIC) is the
lowest concentration of an antimicrobial that will
inhibit the visible growth of microorganisms after
overnight incubation [12, 13]. Standard strain of known
MIC value run with the test is used as the control to
check the reagents and conditions. The main
advantage of this technique is the generation of
quantitative result. The disadvantage includes the
possible errors in preparation of antibiotics solutions
[13].
Preparation of Stock solution: Stock solution can be
prepared using the formula
1000/P x V x C= W where P=Potency given by the
manufacturer in relation to the base, V= Volume in
ml required, C=Final concentration of solution
(multiples of 1000), W= Weight of the antimicrobial
to be dissolved in the volume V. Example: For
making 10 ml solution of the strength 10mg/ml from
powder base whose potency is 500 mg/g, the

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quantities of the antimicrobials required is W =
1000/500 x 10 x 10 = 200 mg.
Micro broth dilution test: This is a miniaturization
and mechanization of the macro broth dilution test. In
this test a small, disposable, polystyrene micro –
dilution panel “Figure 5”, is used. The standard panel
contain 96 wells (8X12) having volume of 100 μL
each. Approximately 12 antibiotics can be tested in a
range of 8 two fold dilutions in a single panel. The
procedure followed in this method is same as
mentioned in macro broth dilution test [13 - 16]. The
MIC is expressed as the highest dilution which
inhibited growth judged by lack of turbidity in the
well. Minimum inhibitory concentrations can be
determined by reading manually or by using
automated turbidity readers [13]. The reproducibility
and availability of the preprepared panel are the
advantages of this method.
Agar dilution method: Agar dilution involved an
incorporation of different concentrations of the
antimicrobial agent into a nutrient agar medium
followed by swabbing of the standardized number of
microbial cells with the sterile cotton swab on to the
surface of the agar plate [13 - 16]. The plates were
incubated for 18 – 24 h at 35 – 37 0C and examined
for the growth inhibited zones. The MIC is expressed
as the highest dilution which inhibited growth by
measuring the zone of inhibition. Agar dilutions are
most often prepared in petri dishes and have
advantage that it is possible to test several organisms
on each plate. The dilutions are made in a small
volume of water and added to agar which has been
melted and cooled to not more than 60oC. Blood may
be added and if ‘chocolate agar’ is required, the
medium must be heated before the antibiotic is
added. The pH of the agar must be between 7.2 and
7.4 at room temperature. Supplemental cations must
not be added to the agar. It may be supplemented
with 5% defibrinated sheep blood or lysed horse
blood. The reproducibility of the results and
satisfactory growth of most nonfastidious organisms
can be expected advantages from agar dilution
method. However, its disadvantages include the
labor required to prepare the agar dilution plates and
their relatively short shelf life.
E Test (Diffusion and dilution): The principle of E
test (also known as Epsilmeter test) method is based
on antimicrobial concentration gradient in an agar
plate. An ‘E’ in E test refers to the Greek symbol
epsilon ().The E test (bioMerieux AB Biodisk) is a
quantitative method for antimicrobial susceptibility
testing applies both the dilution and diffusion of
antibiotic into the medium. A predefined stable
antimicrobial gradient is present on a thin plastic
inert carrier strip. These strips are impregnated on the
underside with a dried antibiotic concentration
gradient and are labeled on upper surface with a
concentration scale “Figure 6”. When this E test strip
was placed onto an inoculated agar plate, there was
an immediate release of the drug. Following
overnight incubation, a symmetrical inhibition ellipse
was produced. The MIC value over a wide
concentration range (˃ 10 dilutions) is determined by
intersection of the lower part of the ellipse shaped
growth inhibition area with the test strip. Some
investigators have reported an excellent correlation
between E-test results and broth dilution or agar
dilution methods [16].
E test have been used to determine MIC for
fastidious organisms like S. pneumoniae, ß-
hemolytic streptococci, N.gonorrhoeae, Haemophilus
sp. and anaerobes. It can also be used for
Nonfermenting Gram Negative bacilli (NFGNB) for
eg-Pseudomonas sp. and Burkholderia pseudomallei
[17 - 19]. The cost the E test is little more when
compared to disk diffusion method. However the E
test is simple, accurate, and reliable and is also used
to determine the Minimum Inhibitory Concentration
(MIC) of antifungal agents and antimycobacterial
agents [20].
Current test methods and future perspective
The antibiotic susceptibility testing methods
discusses here in this article provide reliable results
when procedures are followed as defined by the CLSI
or by the manufacturers of the commercial products.
However, there is considerable opportunity for
improvement in the area of rapid and accurate
recognition of bacterial resistance to antibiotics.
There is a need for development of new automated
instruments that could provide faster results and also
save money by virtue of lower reagent costs and
reduced labor requirements [21].
Conflict of interests: The author declared no conflict
of interest.

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Figure 1: Kirby Bauer disk disc diffusion method
Figure 2: Agar well diffusion method
Figure 3: Stokes Disc Diffusion method

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Figure 4: Tube or Macro broth dilution test
Figure 5: A broth microdilution panel containing 96 wells
Figure 6: E Test gradient diffusion method

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