Problematic clinical isolates of Pseudomonas
aeruginosa from the university hospitals in Sofia,
Bulgaria: current status of antimicrobial resistance
and prevailing resistance mechanisms
Tanya Strateva,1Vessela Ouzounova-Raykova,1Boyka Markova,2
Albena Todorova,3Yulia Marteva-Proevska2and Ivan Mitov1
1Department of Microbiology, Medical University of Sofia, 2 Zdrave Street, 1431 Sofia, Bulgaria
2Laboratory of Clinical Microbiology, Alexander University Hospital, Medical University of Sofia,
1 Georgi Sofiiski Blvd, 1431 Sofia, Bulgaria
3Laboratory of Molecular Pathology, University Hospital of Obstetrics and Gynecology,
Medical University of Sofia, 2 Zdrave Street, 1431 Sofia, Bulgaria
Received 4 October 2006
Accepted 13 March 2007
A total of 203 clinical isolates of Pseudomonas aeruginosa was collected during 2001–2006
from five university hospitals in Sofia, Bulgaria, to assess the current levels of antimicrobial
susceptibility and to evaluate resistance mechanisms to antipseudomonal antimicrobial agents.
The antibiotic resistance rates against the following antimicrobials were: carbenicillin 93.1%,
azlocillin 91.6%, piperacillin 86.2%, piperacillin/tazobactam 56.8%, ceftazidime 45.8%,
cefepime 48.9%, cefpirome 58.2%, aztreonam 49.8%, imipenem 42.3%, meropenem 45.5%,
amikacin 59.1%, gentamicin 79.7%, tobramycin 89.6%, netilmicin 69.6% and ciprofloxacin
80.3%. A total of 101 of the studied P. aeruginosa isolates (49.8%) were multidrug resistant.
Structural genes encoding class A and class D b-lactamases showed the following frequencies:
blaVEB-133.1%, blaPSE-122.5%, blaPER-10%, blaOXA-groupI41.3% and blaOXA-groupII8.8%.
IMP- and VIM-type carbapenemases were not detected. In conclusion, the studied clinical strains
of P. aeruginosa were problematic nosocomial pathogens. VEB-1 extended-spectrum
b-lactamases appear to have a significant presence among clinical P. aeruginosa isolates from
Sofia. Carbapenem resistance was related to non-enzymic mechanisms such as a deficiency of
OprD proteins and active efflux.
Pseudomonas aeruginosa is responsible for 10–15% of
nosocomial infections worldwide (Blanc et al., 1998). The
infections are frequently difficult to treat because of both
the natural resistance of the species and its remarkable
ability to acquire further resistance mechanisms to multiple
groups of antimicrobial agents. P. aeruginosa represents a
phenomenon of antibiotic resistance, demonstrating prac-
tically all known enzymic and mutational mechanisms of
bacterial resistance. These mechanisms are often present
simultaneously, conferring combined resistance to many
strains (McGowan, 2006).
Multidrug-resistant strains of P. aeruginosa (resistant to at
least three of the following antimicrobials: ceftazidime,
imipenem, gentamicin and ciprofloxacin) are often isolated
among patients suffering from nosocomial infections,
particularly those receiving intensive care treatment
(Tassios et al., 1997). The increasing rate of P. aeruginosa
strains in a wide spectrum of clinical settings determines
them as emerging pathogens, especially in intensive care
units (ICUs), and justifies the necessity for antimicrobial-
The aim of this study was to assess the current levels of
antimicrobial susceptibility and to evaluate the resistance
mechanisms to antipseudomonal antimicrobial agents
among problematic clinical isolates of P. aeruginosa
collected from five university hospitals in Sofia, Bulgaria.
Abbreviations: AAC, aminoglycoside acetyltransferase; ANT, aminogly-
coside nucleotidyltransferase; ESBL, extended-spectrum b-lactamase;
ICU, intensive care unit; LRTI, lower respiratory tract infection; MBL,
metallo-b-lactamase; URTI, upper respiratory tract infection.
The GenBank/EMBL/DDBJ accession nos for the P. aeruginosa
blaVEB-1and blaPSE-1gene sequences are DQ333895 and M69058,
Journal of Medical Microbiology (2007), 56, 956–963
95646986G2007 SGMPrinted in Great Britain
Bacterial isolates. A collection of 203 non-duplicate, problematic
clinical isolates of P. aeruginosa (resistant to one or more of the
following groups of antimicrobials: third- or fourth-generation
cephalosporins, carbapenems, aminoglycosides and fluoroquino-
lones) were used in the present study. The strains were collected
during the period 2001–2006 from in-patients of different types of
ward in five university hospitals in Sofia: surgical, orthopaedic,
internal, paediatric, neurological and ICUs. The isolates were
obtained from urine (79), tracheal aspirates (27), sputum (14),
bronchial lavage (12), pleural fluid (2), surgical wounds or abscesses
(30), drainages (9), blood (9), nose (9), throat (7), ear (1), rectal
swabs (2) and bile (2). Bacterial identification was performed using a
BBL Enteric/Nonfermenter ID system (Becton Dickinson).
Antimicrobial-susceptibility testing. The susceptibility of the
investigated P. aeruginosa isolates to 17 antimicrobial agents was
determined by the disc diffusion method on Mueller–Hinton II agar
plates (Becton Dickinson) using antibiotic-containing discs provided
by Becton Dickinson, Mast Diagnostics and Bul Bio, and was
interpreted according to the National Committee for Clinical
Laboratory Standards (NCCLS) (now the Clinical and Laboratory
Standards Institute) 2004 recommendations (NCCLS, 2004). Control
strains included P. aeruginosa ATCC 27853 and Escherichia coli ATCC
Phenotypic methods for detection of resistance mechanisms
to antimicrobial agents
Detection of group 1 inducible b-lactamases. The prevalence of
inducible AmpC b-lactamase (molecular class C, functional group 1)
(Bush et al., 1995) in the studied strains of P. aeruginosa was
investigated using a disc approximation test method (Sanders &
Sanders, 1992). A ceftazidime (30 mg) disc was placed at a distance of
20 mm (centre to centre) from an imipenem (10 mg) disc on a
Mueller–Hinton II agar plate inoculated with a suspension of the test
organism, adjusted to a McFarland no. 0.5 tube. After overnight
incubation, distinct flattening of the inhibitory zone around the
ceftazidime-containing disc on the side nearest to the imipenem disc
was taken to indicate the presence of inducible AmpC b-lactamase.
Screening for extended-spectrum b-lactamases (ESBLs). The
presence of ESBLs was investigated by the double disc synergy test
(Jarlier et al., 1988). Ceftazidime (30 mg), cefepime (30 mg), cefpirome
(30 mg) and aztreonam (30 mg) discs were placed next to an
amoxicillin/clavulanic acid (20/10 mg)-containing disc at a distance of
20 mm (centre to centre) on a Mueller–Hinton II agar plate
inoculated with the test organism. After overnight incubation at 37 uC,
an enhancement of the inhibition zone around at least one of these
discs toward the clavulanate-containing disc indicated the presence of
ESBLs. All studied strains were tested additionally by a disc diffusion
method with imipenem (10 mg) and ceftazidime (30 mg) discs for the
presence of synergism (Weldhagen et al., 2003).
Screening for metallo-b-lactamases (MBLs). The presence of
Ambler class B MBLs (Bush et al., 1995) was studied using the
modified Hodge test (Lee et al., 2001).
Detection of presumptive aminoglycoside-modifying enzymes.
This test was performed according to the substrate profile, as
described by the Aminoglycoside Resistance Study Groups (1994).
PCR amplification and sequencing of b-lactamase genes. Total
DNA from P. aeruginosa isolates was extracted by boiling. The
detection of blaVEB-1, blaPER-1, blaPSE-1, blaOXA-groupI, blaOXA-groupII,
blaIMP-like and blaVIM-like genes in the investigated strains was
performed by PCR with the specific primers (Alpha DNA) listed in
Table 1. PCR was carried out with 2 ml template DNA, 0.25 mM each
primer, 0.2 mM deoxyribonucleoside triphosphates, 16 reaction
buffer, 2 mM MgCl2and 1.5 U Prime Taq DNA polymerase (GENET
BIO) in a total volume of 25 ml. The DNA was amplified in a Techgen
PCR thermocycler (Techne) using the following protocol: initial
denaturation (94 uC for 5 min), followed by 30 cycles of denaturation
(94 uC for 45 s), annealing (50–64 uC, from 45 s to 1 min) and
extension (72 uC, from 45 s to 1 min), with a single final extension of
7 min at 72 uC. PCR products were separated in 1% agarose gel for
50 min at 150 V, stained with ethidium bromide (0.5 mg ml21) and
detected by a UV transillumination (wavelength 312 nm). The
amplified genes were identified on the basis of fragment size (shown
in Table 1). Selected VEB-1 and PSE-1 PCR products were purified
with ExoSAP-IT reagent (Amersham Biosciences). Sequencing
Table 1. Oligonucleotides used as primers for amplification and sequencing
64358Naas et al. (2000)
PER-1 ESBL92550Claeys et al. (2000)
PSE-1 699 54De Champs et al. (2002)
OXA group I75958Naas et al. (2000)
OXA group II70164De Champs et al. (2002)
IMP-type MBLs 587 56Nordmann & Poirel (2002)
VIM-type MBLs51060 Nordmann & Poirel (2002)
*F, Forward; B, backward.
DY5C or T; M5A or C.
Antimicrobial resistance in clinical P. aeruginosa
reactions were performed using the same blaVEB-1-and blaPSE-1-
specific primers and a BigDye terminator v3.1 kit (Applera) in an
automated sequencer (ABI 310 sequence genetic analyser; Applied
Biosystems). The nucleotide and deduced amino acid sequences were
analysed with software available from the National Center for
Biotechnology Information (http://www.ncbi.nlm.nih.gov).
Statistical analysis. Student’s t-test was used to assess differences in
resistance rates. A P value below 0.05 was considered to be statistically
RESULTS AND DISCUSSION
The antimicrobial-resistance testing results are presented in
Table 2. The established antimicrobial resistances, in
increasing order,were to:
racillin/tazobactam. Polymyxin B remained active against
all isolates. The antimicrobial resistance to antibiotics of
the investigated problematic strains of P. aeruginosa was
higher than the mean P. aeruginosa resistance found in
Bulgaria in 2003, according to data from the national
program BulSTAR: 45.8 vs 24.5% to ceftazidime, 42.3 vs
8.3% to imipenem, 59.1 vs 24.9% to amikacin, 79.7vs
38.7% to gentamicin and 80.3vs 30.7% to ciprofloxacin
(Petrov et al., 2005). Approximately half of our isolates
(49.8%) were multidrug resistant.
The comparative temporary resistance rates among the
studied P. aeruginosa isolates are shown in Fig. 1(a, b). The
strains isolated during 2004–2006 were significantly more
resistant (P,0.001) than those obtained during 2001–2003
to the following antibiotics: ceftazidime (79.5 vs 20.0%),
cefepime (75.0vs 26.9%), cefpirome (81.6vs 7.6%),
aztreonam (73.9vs 31.0%), imipenem (59.1vs 31.3%),
47.8%). Thus, these results demonstrated increasing
resistance rates to extended-spectrum antipseudomonal
cephalosporins, carbapenems, monobactams and amikacin
in P. aeruginosa isolates from the university hospitals in
Sofia during the last 3 years.
The strains of P. aeruginosa from ICUs were more resistant
to antibiotics than the overall studied strains, except to
cefpirome and aztreonam (Table 2). The ICU isolates were
significantly more resistant to meropenem (61.4%) than
all investigated isolates as a whole (45.5%, P,0.05), which
is related to the widespread use of meropenem for the
treatment of life-threatening infections in ICUs.
The antimicrobial resistance in P. aeruginosa varied among
different clinical specimens (Table 2). The P. aeruginosa
isolates from in-patients with lower respiratory tract
infections (LRTIs) were more resistant to piperacillin than
(P,0.05). The strains of P. aeruginosa isolated from in-
patients with LRTIs and upper respiratory tract infections
(URTIs) were more resistant to piperacillin/tazobactam
than those from urine and wounds (P,0.01 and P,0.001,
respectively). The observed resistance rate to ceftazidime in
P. aeruginosa from wounds and drainages was lower than
that in URTI isolates (P,0.05). URTI strains of P.
aeruginosa were more resistant to cefpirome than LRTI
isolates (P,0.05). The antibiotic resistance of P. aeruginosa
Table 2. Antimicrobial resistance as percentage of isolates from different sources among 203 isolates of P. aeruginosa
Urine (n579) LRTI (n555) Wounds and
URTI (n517)Blood culture
*Antimicrobial resistance of all strains of P. aeruginosa, including two rectal swab and two bile isolates.
T. Strateva and others
958Journal of Medical Microbiology 56
from urine samples towards imipenem and meropenem
was lower than that among URTI strains (P,0.02 and
P,0.01, respectively). URTI isolates were significantly
more susceptible than the P. aeruginosa isolates from
urine, LRTIs, and wounds and drainages to the following
(P,0.001), tobramycin (P,0.001), netilmicin (P,0.01,
P,0.001 and P,0.01, respectively) and ciprofloxacin
A total of 104 of the 203 investigated problematic P.
aeruginosa isolates (51.2%) showed a ‘penicillinase produc-
tion phenotype’ (resistance to carboxypenicillins and
ureidopenicillins, and susceptibility to ceftazidime) (Bert
et al., 2003). These strains were presumptive producers of
narrow-spectrum b-lactamases. Additionally, they expressed
an inducible AmpC b-lactamase (cephalosporinase).
The ceftazidime resistance rate was 45.8%. A total of 57 of
the strains of the 203 studied P. aeruginosa isolates (28.1 %)
were resistant toextended-spectrum
including ceftazidime, and were characterized as presump-
tive producers of ESBLs according to the double disc
synergy test. These strains also displayed in vitro synergism
between imipenem and ceftazidime, typical of the produ-
cers of clavulanic acid- and tazobactam-inhibited ESBLs
from molecular class A, such as VEB-, PER- or GES-type
(Weldhagen et al., 2003).
Twelve strains of P. aeruginosa (5.9%) were resistant to all
b-lactams except carbapenems, and showed a negative
result in the double disc synergy test. In these isolates,
related mainly to the overproduction of a chromosomal
AmpC cephalosporinase from molecular class C.
Of the 203 P. aeruginosa isolates, 12 (5.9%) possessed a
OprD2mutant phenotype (resistant only to imipenem and
Carbapenem resistance is mostly due to OprD deficiency
and is independent of susceptibility towards other b-lactam
agents (Livermore, 2001). This resistance mechanism
demands continued expression of the chromosomal
AmpC b-lactamase (Livermore, 1992).
Overproduction of active efflux systems with wide
substrate profiles was the prevailing resistance mechanism
in eight P. aeruginosa isolates (3.9%). In our study, the
presumptive efflux systems were: MexA–MexB–OprM
associated with decreased susceptibility or resistance to
all b-lactams, except imipenem, and with decreased
susceptibility or resistance to quinolones (nalB or nalC
mutants) (Llanes et al., 2004), and MexC–MexD–OprJ
conferring resistance to fourth-generation cephalosporins
(cefepime and cefpirome) and resulting from mutation in
nfxB (Poole et al., 1996).
Sixty strains of P. aeruginosa (29.6%) were resistant to all
b-lactams, including carbapenems, and thus could be
related to a phenotype of Ambler class B MBL-producing
strains (Nordmann & Poirel, 2002). All carbapenem-
resistant strains of P. aeruginosa showed a negative
Hodge test and therefore were not producers of MBLs.
Most probably, the resistance to b-lactams resulted from a
combination of different mechanisms: OprD deficiency,
derepression of chromosomal AmpC cephalosporinase,
ESBL production and overexpression of active efflux
One hundred and twenty isolates out of the studied strains
of P. aeruginosa (59.1%) were resistant to amikacin and
69.6–89.6% to the other aminoglycosides (netilmicin,
mechanism of resistance to these antimicrobials involves
enzymic modification by aminoglycoside acetyltransferases
(AACs), aminoglycoside nucleotidyltransferases (ANTs) or
aminoglycoside phosphotransferases (Poole, 2005). The
prevailing phenotypes of aminoglycoside resistance in our
strains were: (i) amikacin+gentamicin+tobramycin+ne-
tilmicin (49.9%), associated with AAC (69)-I±ANT (299);
(ii) gentamicin+tobramycin+netilmicin (14.1%), asso-
ciated with AAC (3)-V or ANT (299)+AAC (3)-Ia; and
(iii) amikacin+gentamicin+tobramycin (14.1%), related
Fig. 1. Comparative temporary antimicrobial resistance in the
studied P. aeruginosa isolates. m, 2001–2003 (n5115); h,
Carbenicillin; AZL, azlocillin; PIP, piperacillin; TZP, piperacillin+
tazobactam; CAZ, ceftazidime; CPZ, cefoperazone; FEP, cefe-
pime; CPO, cefpirome; ATM, aztreonam; IMP, imipenem; MEM,
meropenem. (b) Resistance to aminoglycosides and fluoroquino-
lones. AMK, Amikacin; GEN, gentamicin; TOB, tobramycin; NET,
netilmicin; CIP, ciprofloxacin.
Antimicrobial resistance in clinical P. aeruginosa
to aminoglycoside phosphotransferase (39)-VI+ANT (299)
(Aminoglycoside Resistance Study Group, 1994).
One hundred and sixty three (80.3%) of the isolates were
resistant to ciprofloxacin. The most important mechanisms
of quinolone resistance are structural alterations of the
primary or secondary targets because of chromosomal
point mutations in gyrA/gyrB or parC/parE genes, respec-
tively, followed by an active efflux of these antimicrobial
agents (Hooper, 2001).
A molecular genetic study was carried out for the presence
of b-lactamases belonging to different molecular classes. A
total of 160 isolates were investigated, and 53 (33.1%) were
found to be VEB-1 producers. The sequence of blaVEB-1
amplified from different selected isolates was identical for
all isolates and 100% identical to the known veb-1
sequence (GenBank accession no. DQ333895). The fre-
quency of VEB-1 ESBLs among the ceftazidime-resistant P.
aeruginosa was 57.0% (53/93). A total of 36 of the 160
isolates (22.5%) produced PSE-1 enzyme. Selected PSE-1
PCR products showed 100% identity to blaPSE-1(GenBank
accession no. M69058). The frequency of OXA group I and
OXA group II b-lactamases was 41.3% (66/160) and 8.8%
The distribution of the Ambler class A and D b-lactamases
among the investigated strains of P. aeruginosa is presented
in Table 3. As shown, the relative proportion of b-
lactamase-producing strains of P. aeruginosa (66.8%) was
higher than the proportion of b-lactamase-non-producing
strains (33.1%). An analogous study carried out recently in
Korea established that b-lactamase-non-producing strains
of P. aeruginosa were more widespread than producers
(74.6vs 25.4%; Lee et al., 2005).
In our investigation, the b-lactamase producers were
mostly present as VEB-1+OXA group I (20.0%), followed
by PSE-1 (13.1%) and OXA group I (12.5%). The
frequency of the Ambler class A b-lactamases (55.6%)
was approximately equal to the frequency of class D
b-lactamases (50.1%). In comparison, class D OXA-type
enzymes were detected more frequently than class A in P.
aeruginosa from Korea (21.0 vs 6.3%; Lee et al., 2005), in
contrast to the data from Europe (31.3 vs 64.9%; Bert et al.,
A high frequency of distribution of ESBLs in the
ceftazidime-resistant isolates of P. aeruginosa was estab-
lished in our study. In all the university hospitals
monitored in Sofia, widespread dissemination of blaVEB-1
in clinical isolates of P. aeruginosa was found. Recently,
Bachvarova et al. (2005) reported a significantly lower
(P,0.01) rate of prevalence of VEB-1-type b-lactamases
among ceftazidime-resistant strains of P. aeruginosa
obtained from distinct regions of Bulgaria during 1998–
2003 than that determined in our study (36.8 vs 57.0%).
Thus, the observed trend towards an increasing rate of
VEB-1-producing P. aeruginosa strains in Bulgaria relates
to the last 2 years. VEB-1 and VEB-1-like enzymes are
widespread in Asia (Thailand, Kuwait, India and China)
(Weldhagen et al., 2003; Girlich et al., 2002; Poirel et al.,
2001), but in European countries have been detected only
in France (Naas et al., 1999).
A total of 42 (26.3%) of the 160 isolates studied possessed
both VEB-1 and OXA group I enzymes. It is likely that the
strains produced the narrow-spectrum OXA-10 from OXA
group I (Sanschagrin et al., 1995), encoded by a gene
located on class 1 integron In50, as well as blaVEB-1(Girlich
et al., 2002). Lee et al. (2005) reported that OXA-10 was the
most prevalent enzyme (13.5%) in Korea in 2002.
PSE-1 b-lactamases belonging to Ambler class A and
functional group 2c (Bush et al., 1995) were detected in
22.5% of the investigated strains. In 2002, Nordmann
(2002) reported 11% CARB-producing strains of P.
aeruginosa in France, 90% of which were PSE-1.
Our study did not reveal blaPER-1, in contrast to the
widespread detection of these genes in Europe. Epidemics
caused by PER-1-producing P. aeruginosa have been
reported previously in Turkey and Italy (Vahaboglu et al.,
1997; Luzzaro et al., 2001). Isolates of P. aeruginosa with
Table 3. Prevalence of Ambler class A and D b-lactamases in 160 P. aeruginosa isolates
Class Type of b-lactamase No. (%) of isolates
Class A PSE-1
VEB-1 and PSE-1
OXA group II
OXA group I
VEB-1 and OXA group I
VEB-1, PSE-1 and OXA group I
VEB-1, PSE-1, OXA group I and OXA group II
PSE-1 and OXA group I
PSE-1 and OXA group II
VEB-1, OXA group I and OXA group II
T. Strateva and others
960Journal of Medical Microbiology 56
PER-1 enzymes have also been observed in France, Belgium
and Poland (De Champs et al., 2002; Claeys et al., 2000;
Empel et al., 2005).
The established frequency of OXA group II b-lactamases
comprising OXA-2, -3, -15 and -20 (Sanschagrin et al.,
1995) was the lowest in our research. It is likely that the
oxacillinases from group II were predominantly narrow
spectrum, such as OXA-2 or -3, as these enzymes were
detected mainly in ceftazidime-susceptible strains of P.
aeruginosa. In comparison, OXA group II enzymes were
disseminated among 2.3% of the studied P. aeruginosa
isolates in Korea and all strains were determined as OXA-2
producers (Lee et al., 2005). The frequency of blaOXA-groupII
in our strains of P. aeruginosa (8.8%) was similar to the
dissemination rate of OXA group II b-lactamases in France
during 1994–1999 (9.9%) (Bert et al., 2002).
Carbapenem-hydrolysing IMP- and VIM-type metalloen-
zymes belonging to Ambler class B were not detected in
this study. The investigated carbapenem-resistant strains of
P. aeruginosa from Sofia did not harbour blaVIM-likegenes,
although the detection of these genes is widespread,
especially in neighbouring countries such as Greece and
Turkey (Tsakris et al., 2000; Mavroidi et al., 2000;
Pournaras et al., 2002; Bahar et al., 2004). The carbapenem
resistance was related to non-enzymic mechanisms such as
OprD deficiency and active efflux.
The comparative antimicrobial resistances of b-lactamase-
producing and b-lactamase-non-producing P. aeruginosa
are summarized in Table 4. The b-lactamase producers
were significantly more resistant than non-producers to
ceftazidime, cefepime, cefpirome, aztreonam, amikacin,
tobramycin and ciprofloxacin (P,0.001) and to gentamicin
cephalosporins, aztreonam and carbapenems among b-
lactamase-producing strains of P. aeruginosa were lower than
those in non-b-lactamase producers and were similar to the
susceptibilities in analogous P. aeruginosa isolates from
Korea in 2002 (Lee et al., 2005). Moreover, in our study and
the Korean study, the cross-class resistance to aminoglyco-
sides and ciprofloxacin was significantly higher in classA and
D b-lactamase-producing P. aeruginosa. As described pre-
viously, VEB-1 was the first class A enzyme found to be
encoded by an integron-located gene cassette (Poirel et al.,
1999). In the blaVEB-1-containing integrons of P. aeruginosa,
the veb-1 cassette is often associated with aminoglycoside
resistance gene cassettes (Girlich et al., 2002).
In conclusion, the studied clinical strains of P. aeruginosa
were problematic nosocomial pathogens and half were
found to be multidrug resistant. From 2001 to 2006, the
rates of resistance to third- or fourth-generation cephalos-
porins, monobactams, carbapenems and amikacin showed
significant increases among the investigated P. aeruginosa
strains from the monitored hospitals. The interpretation of
the phenotypic patterns of antimicrobial susceptibility
showed a variety of resistance mechanisms, from which the
prevalent were expression of an inducible AmpC cepha-
losporinase, production of ESBLs and a combination of
mechanisms conferring resistance to multiple groups of
antimicrobials. The carbapenem resistance was not related
to enzymic hydrolysis by Ambler class B MBLs. The
clavulanic acid-inhibited VEB-1-type ESBLs appear to have
a significant presence among P. aeruginosa isolates in the
university hospitals in Sofia and cause serious impediments
in antimicrobial treatment and difficulties in limiting their
dissemination in Bulgaria.
Table 4. Comparison of antimicrobial resistance (%) between the class A and/or class D b-
lactamase-producing and -non-producing P. aeruginosa isolates
Antimicrobial resistance in clinical P. aeruginosa
This work was supported by grant 05-2005 of the Medical University
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