Relation between Resistance to Antipseudomonal β-Lactams and ampC and mexC Genes of Pseudomonas aeruginosa

Background: In order to select a better antibiotic choice for treatment of Pseudomonas aeruginosa infections, this study was conducted to determine the frequency of resistance to some antipseudomonal β-lactams in P. aeruginosa isolates from patients in Tehran, Iran. In addition, the relation between presence of genes known to be responsible for resistance to β-lactams (ampC, mexC1,2,and mexC3,4 genes) and resistance phenotype among P. aeroginosa isolates was evaluated. Methods: P. aeruginosa strains were isolated and identified by routine methods and PCR for oprL gene. Disk diffusion method was employed to determine the antimicrobial susceptibility pattern according to CLSI recommendations. PCR was used to detect the resistance genes. Results: Among 100 isolates of P. aeruginosa, 82% had ampC, 86% mexC1,2and 89% mexC3,4 genes and combinations of these genes were seen in most of isolates and only 3% of isolates had none of these genes. Resistance to mezlocillin, cefepime, ceftazidime and piperacillin/ tazobactam was seen in 46%, 41%, 36% and 29% of isolates, respectively. Significant relation (P value ≤0.05 by Chi-square or Fisher Exact test) was observed between the presence of ampC gene and resistance to all the studied β-lactams in this study. No relation was observedfor mexC genes,although many ofisolates containing these two genes were phenotypically resistant.  Conclusion: This study had shown for the first time, the presence of ampC and mexC genes in significant percent of clinical isolates of P. aeruginosa in Tehran, Iran, and relation between presence of ampC gene and resistance to β-lactams.