IN VITRO COMPARISON OF DISK DIFFUSION METHOD AND AGAR DILUTION METHOD FOR SENSITIVITY OF POLYMYXIN B AGAINST MULTI DRUG RESISTANT ACINETOBACTER BAUMANNII

Objective: To compare the in vitro disk diffusion method and agar dilution method for sensitivity of Polymyxin B against multi drug resistant Acinetobacter baumannii. Study Design: Comparative cross sectional. Place and Duration of Study: The study was carried out in the department of Microbiology, Armed Forces Institute of Pathology (AFIP), Rawalpindi from 1st Dec 2016 to 30th Dec 2017. Methodology: Total 253 clinical specimens received from intensive care units with yielded growth of multidrug resistant Acinetobacter baumannii were evaluated for Polymyxin B susceptibility. Both Kirby bauer disk diffusion and agar dilution methods were compared with a reference method, broth microdilution. Results: Among 253 multidrug resistant isolates, 180 (71%) were extensively drug resistant and 221 (87%) were carbapenem resistant. Comparison of the disk diffusion and the MIC method by Agar dilution showed 100% correlation. 251 isolates were sensitive and 2 were resistant to Polymyxin B with MICs and disk zone diameters within the range recommended by the CLSI (2014-17). MIC 50 and MIC 90 of Polymyxin B were found to be 0.5 and 1μg/ml with 99.2% susceptibility. Conclusion: Both disk diffusion and agar dilution can be used together as initial screening methods in low income countries for susceptibility reporting of polymyxins. They are simple, reliable and economical. Also hetero resistance shown by some Acinetobacter baumanii strains can easily be picked up by agar dilution method.