ABSTRACT
             Background and Objectives: Emergence and spread of multidrug-resistant (MDR) and extensively-drug resistant (XDR) Pseudomonas aeruginosa strains could complicate antipseudomonal chemotherapy. Dissemination of resistance genes, such as β-lactamases encoding genes by horizontal gene transfer can lead to development of multi-drug resistance in P. aeruginosa. The purpose of this study was to investigate the latest resistance patterns in MDR and XDR strains and evaluate Ambler class A β-lactamase gene distribution in P. aeruginosa clinical isolates.
             Methods: One hundred molecularly and biochemically identified P. aeruginosa strains isolated from different clinical specimens were tested for sensitivity to 17 antibiotics using the Kirby-Bauer disk diffusion method. PCR was performed to detect bla _TEM-1, bla _SHV-1, bla _REP-1 and bla _VEB-1 genes. Results were analyzed using SPSS and NTSYSpc softwares. 
             Results: Based on the results of antibiogram, the highest rate of resistance was observed against amikacin (100%), aztreonam (83%), ceftazidime (55%), cefepime (55%) and netilmicin (48%). In addition, the frequency of MDR and XDR isolates was 95% and 5%, respectively. The bla_SHV-1, bla _TEM-1, bla _PER-1 and bla _VEB-1 genes were detected in 31%, 24%, 13% and 10% of the isolates, respectively.
             Conclusion: Antibiotic resistance to β-lactam antibiotics and frequency of β-lactamase genes were relatively high in the study area. We also found that a significant proportion of XDR strains with different antibiotic resistance profile is isolated from tracheal specimens.
             KEYWORDS: Pseudomonas aeruginosa, Beta-Lactamase, Multidrug Resistant, Extensively Drug Resistant.