Emergence of oxacillinase-Mediated Resistance to carbapenem in Acinetobacter baumannii Clinical Isolates

Background and Objectives: Carbapenem is a selective antibiotic for the treatment of infections caused by Acinetobacter baumannii. The presence of the blaOXA genes in A. baumannii contributes to carbapenem resistance. The present study aimed to determine the prevalence of blaOXA-type carbapenemases in clinical isolates of Acinetobacter baumannii. Materials and Methods: This study was conducted on 65 clinical isolates of Acinetobacter baumannii. Antimicrobial susceptibility test was performed by disc diffusion agar method for ceftazidime, cefepime, piperacillin/tazobactam, ampicillin/sulbactam, imipenem, meropenem, gentamicin, tobramycin, ciprofloxacin, levofloxacin, tetracycline and minocycline. Minimum Inhibitory Concentration (MIC) of isolates was determined for the imipenem, meropenem, polymyxin B, colistin and tigecycline using standard microbroth dilution method. The frequency of the genes encoding oxacillinase, including OXA-143-like ،OXA-23-like ،OXA-58-like ،OXA-24-like ،OXA-51-like and OXA-235-like in carbapenem resistant Acinetobacter baumannii (CRAB) isolates was studied by PCR. Results: The resistance patterns of A. baumannii isolates for ceftazidime, cefepime, piperacillin-tazobactam, ampicillin-sulbactam, imipenem, meropenem, gentamicin, tobramycin, ciprofloxacin, levofloxacin, tetracycline and minocycline were 96.92%, 92.30%, 96.92,%, 7.69%, 96.92%, 96.92%, 73.84%, 60%, 93.84%, 72.30%, 27.69% and 0%, respectively. The resistance patterns to the seven different antibiotic classes in A. baumannii isolates showed that the prevalence rates of multidrug-resistant (MDR), extensively drug-resistant (XDR) and pandrug resistant (PDR) were 92.96%, 39.62% and 0%, respectively. The minimum inhibitory concentration of the isolates showed that the resistance rates to the imipenem, meropenem, polymyxin B, colistin and tigecycline were 100%, 100%, 0%, 12.7% and 36.51%, respectively. The presence rates of genes encoding oxacillinase such as blaOXA-51-like, blaOXA-58-like, blaOXA-23-like, blaOXA-24-like, blaOXA-143-like, and blaOXA-235-like were 100%, 0%, 0%, 74.61%, 42/62%, 0%, and 0%, respectively.Conclusion: The present study indicated that the frequency of multidrug-resistant (MDR) is high in clinical isolates of Acinetobacter baumannii. This is a serious warning for public health organizations and healthcare systems. These in vitro results show that the polymyxin B has a good antimicrobial activity against Acinetobacter baumannii.