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 β-lactamases showed the following frequencies: bla _VEB-1 33.1 %, bla _PSE-1 22.5 %, bla _PER-1 0 %, bla _OXA-groupI 41.3 % and bla _OXA-groupII 8.8 %. 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 β-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.

A total of 132 ceftazidime-resistant clinical isolates of Pseudomonas aeruginosa were collected during 2001-2005 from 5 university hospitals in Sofia, Bulgaria to assess the current levels of antimicrobial susceptibility and to evaluate resistance mechanisms to beta-lactams. Antimicrobial susceptibilities were detected by a disk diffusion method and E-test. Polymerase chain reaction amplification and sequencing of bla(VEB-1 )and bla(PER-1 )were performed. The antibiotic resistance rates were: to piperacillin 90.2%, piperacillin/tazobactam 52.3%, ceftazidime 94.7%, cefepime 88.6%, cefpirome 98.5%, aztreonam 85.6%, imipenem 66.6%, meropenem 63.6%, amikacin 81.1%, gentamicin 84.8%, tobramycin 89.4%, netilmicin 57.6%, ciprofloxacin 83.4%. Structural genes for VEB-1 extended-spectrum beta -lactamases (ESBLs) were found in 75 (56.8%) of the isolates. PER-1 ESBLs were not detected. The VEB-1-producing strains were more resistant than VEB-1 non-producers to amikacin, gentamicin, tobramycin and ciprofloxacin ( P<0.001). VEB-1 appears to have a significant presence among ceftazidime-resistant P. aeruginosa isolates from Sofia.