• Dheepa Muthusamy, S Sudhishnaa, and Appalaraju Boppe.
• Associate Professor, Department of Microbiology, Government Medical College & ESIC Hospital , Coimbatore, Tamil Nadu, India .
• J Clin Diagn Res. 2016 Sep 1; 10 (9): DC15-DC18.

IntroductionAcinetobacter baumannii (A.baumannii) is rapidly emerging as a potent organism causing a multitude of nosocomial infections. The organism also carries various resistance mechanisms to antibiotics, making treatment more difficult. Very few choices are left, as A.baumannii strains have begun to develop resistance against cephalosporins, aminoglycosides and even carbapenems.AimTo examine the sensitivity pattern of three older antibiotics namely colistin, polymyxin B and rifampicin against carbapenem resistant A.baumannii by disk diffusion method and the sensitivity of colistin alone by Minimum Inhibitory Concentration (MIC) determination by VITEK automated system.Materials And MethodsHundred clinical isolates of carbapenem resistant A. baumannii were tested for sensitivity to colistin, polymyxin B and rifampicin by Kirby-Bauer disk diffusion method. They were also tested for sensitivity to colistin by VITEK 2C (biomérieux) automated microbial identification system. The zone diameters and Minimum Inhibitory Concentration values for the above two methods, respectively were observed and analysed. All the Antibiotic Susceptibility Tests were done according to the CLSI guidelines.ResultsBy Kirby-Bauer disk diffusion method, 78% of the carbapenem resistant strains were found to be sensitive, 12% intermediate sensitive and 10% resistant to colistin. All the isolates were sensitive to polymyxin B and 80% were resistant to rifampicin. By the VITEK automated system, 99% of the isolates were sensitive to colistin (more in number than by disk diffusion method).ConclusionPolymyxins (colistin - polymyxin E and polymyxin B) are the next choice for multidrug resistant serious nosocomial infections like those of A. baumannii, till newer antibiotics are discovered to treat such infections. Rifampicin resistance was found to be very high and hence, is not advised for monotherapy.

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