In Vitro Activity of Polymyxin B Against Carbapenem-Resistant Acinetobacter baumannii

Visanu Thamlikitkul MD*, Surapee Tiengrim MSc**, Chakrapong Seenama BSc*

Affiliation : * Department of Medicine, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand ** Faculty of Medical Technology, Mahidol University, Bangkok, Thailand

Objective : To determine in vitro activity of polymyxin B against carbapenem-resistant Acinetobacter baumannii. 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 carbapenem- resistant 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

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