[Comparison of disc diffusion, E-test, and broth microdilution methods for the determination of resistance to colistin, polymyxin B, and tigecycline in multi-resistant Acinetobacter baumannii isolates]

Abstract

The increased rate of antimicrobial resistance in Acinetobacter baumannii made it necessary to reconsider old antibiotics such as polymyxins and develop new drugs such as tigecycline. The aim of this study was to investigate the susceptibility rates of multi-drug resistant A. baumannii clinical isolates to colistin, polymyxin B, and tigecycline by three different methods in microbiology laboratory of Gaziantep University Research Hospital, between September 2006 and April 2008. A total of 200 A. boumannii strains isolated from various clinical samples (tracheal aspirate, blood, sputum, surgical wound, catheter, pleural fluid, urine, and others) were included to the study. Identification of bacteria was performed by conventional microbiological methods and by an automatized identification system (Vitek 2, bioMerieux, France). Antimicrobial susceptibility pattern of A. baumannii isolates was determined by disc diffusion method and 95 multiple resistant A. baumannii isolates were identified. Susceptibilities of these multiple resistant bacteria to colistin, polymyxin B, and tigecycline were tested with disc diffusion, E-test, and broth microdilution methods. All of the isolates (100%) were sensitive to colistin with all three methods. Ninety-two (96.8%) of them were sensitive to polymyxin B with both disc diffusion and broth microdilution methods, and 90 (94.7%) of them were sensitive to polymyxin B with E-test. Eighty-three (87.4%) of them were sensitive to tigecycline by disc diffusion method, 78 (82.1%) by E-test, and 90 (94.7%) by broth microdilution method. No statistically significant difference was detected for the three methods in terms of susceptibility testing for polymyxin B (p > 0.05). However, while no significant difference wa detected for tigecycline susceptibility testing by disk diffusion and broth microdilution (p > 0.05), statistically significant difference was determined for broth microdilution and E-test methods (p = 0.000). In conclusion, comparison of disc diffusion, E-test, and broth microdilution methods yielded that all three methods were concordant to each other in terms of susceptibility testing of polymyxins. Susceptibility rate to tigecycline was found lower by E-test method than that obtained by other methods. These results emphasized that antimicrobial activities of colistin, polymyxin B and tigecycline against A. baumannii isolates obtained in our hospital were high, however, for tigecycline susceptibility testing against A. boumannii, disk diffusion or broth microdilution methods would rather be preferred.