In vitro activity of methylene blue and eosin methylene blue agar on colistin-resistant A. baumannii: an experimental study

Introduction. Colistin is a last-resort antibiotic used against carbapenem-resistant Acinetobacter baumannii (AB); however, colistin resistance has been reported recently. Methylene blue (MB) is used in microbiology for staining, and in medicine as an antidote drug.Aim. We aimed to investigate the antimicrobial effects of MB and eosin methylene blue (EMB) agar against colistin-resistant AB strains.Methodology. Firstly, a standard strain and AB clinical isolate were included in the study. After determining MICs, two strains were transformed into colistin-resistant forms, using Li's method. At each step, new MICs were determined and subcultures were inoculated onto EMB and sheep blood agar (SBA). Colistin MICs of the subcultures were also determined using Mueller-Hinton agar (MHA) containing MB. Secondly, colistin-resistant clones from 31 multidrug-resistant AB clinical isolates were screened to investigate their susceptibilities to EMB agar.Results. In the first round, MICs of both strains had risen to 64 μg ml-1. Subpopulations with high colistin resistance were inhibited by MB and EMB agar, but could grow well on SBA. In MHA plates containing MB, the MICs decreased to 0.5 μg ml-1 for colistin-susceptible or moderately resistant clones. Additionally, clones with high colistin resistance showed atypical colony morphology on SBA. In the second round, 35 % of the clinical isolates, which had gained resistance to colistin, were inhibited by EMB agar.Conclusion. MB may have inhibitory effects against colistin-resistant AB. Secondly, using only EMB agar for subculturing may cause missing of colistin-resistant strains.