Material and Method: The activity of polymyxin B was determined against 217 strains of carbapenem-resistant A. baumannii collected from different patients by standard agar dilution method and disk diffusion test using polymyxin B disk (300 units). The control strains were E. coli ATCC 25922 and P. aeruginosa ATCC 27853.

Results: The MIC values and inhibition zone diameters of polymyxin B against the quality control bacteria were within the acceptable range. The MIC50 and MIC90 values of polymyxin B against 217 strains of carbapenem-resistant A. baumannii were 0.5 and 1 mg/l, respectively. If the susceptible MIC breakpoint of polymyxin B was ≤2 mg/l, 98.2% of carbapenemresistant A. baumannii strains were susceptible to polymyxin B. If the susceptible MIC breakpoint of polymyxin B was ≤2 mg/l, the sensitivity and the specificity of the inhibition zone diameter of >12 mm were 100% and 75%, respectively. The aforementioned diagnostic parameters gave positive predictive value of 99.5% and negative predictive value of 100% for predicting susceptibility of carbapenem-resistant A. baumannii to polymyxin B by disk diffusion test.

Conclusion: Polymyxin B was very active against carbapenem-resistant A. baumannii. The inhibition zone diameters of >12 mm was accurate enough to determine susceptibility of carbapenem-resistant A. baumannii to polymyxin B. Polymyxin B can be an alternative to or more preferable than colistin for therapy of carbapenem-resistant A. baumannii infections.

Keywords: Polymyxin B, In vitro activity, Acinetobacter baumannii