Abstract

Acinetobacter baumannii is an opportunistic Gram negative coccobacillus that can grow easily in moist as well as dry conditions. During the last decade, A. baumannii emerged as one of the most resistant opportunistic pathogens responsible for nosocomial infections including ventilator associated pneumonia. The bug remains an important and difficult to treat pathogen whose pan-drug resistant nature has created a serious challenge. This has restricted the choice of treatment modalities. Currently, it appears as if all the available antibiotics are failing against this pathogen while single antibiotic therapy is certainly not working anymore. Thus, there is a strong need, thus, to explore new regimens to combat this resistant organism. A wide range of various combinations of drugs should therefore be tested for their synergistic activity against this pathogen. This study was aimed to find some effective combinations against extensively drug resistant (XDR) A. baumannii by combining various antibacterials. The microdilution checkerboard titration method was used for this purpose and fractional inhibitory concentrations (FICs) were calculated. In-vitro synergy was found in polymyxin B-colistin (n = 3; 15%) and polymyxin B-rifampin (n = 3; 15%) combinations. Only additive effect was noted with polymyxin B-doxycycline (n = 12; 60%), polymyxin B-rifampin (n = 11; 55%), and polymyxin B-colistin (n = 13; 65%). However, antagonism was detected in the polymyxin B-rifampin combination in one of the 20 strains evaluated for the purpose. Polymyxin B in combination with rifampin and colistin may be exploited against XDR A. baumannii. Synergy between polymyxin B and colistin have been demonstrated in only 15% of strains, this fully warrants the testing of more combinations.

Key words: Acinetobacter baumannii, extensively drug resistant, fractional inhibitory concentration.

Abbreviation

A. baumannii, Acinetobacter baumannii; API 20NE, analytical profile index 20 Non-Enterobactericeae; ATCC, American type culture collection; FIC, fractional inhibitory concentration; FICI, fractional inhibitory concentration index; MBC, minimal bactericidal concentration; MDR, multi-drug resistant; MIC, minimal inhibitory concentration; PDR, pan-drug resistant; XDR, extensively drug resistant