Can colistin susceptibility results determined in multidrug resistant Gram-negative bacteria be used for polymyxin B?

Introduction. The present study aimed to compare the MIC values of colistin and polymyxin B in fermentative and non-fermentative Gram-negative bacteria and to examine whether the susceptibility results determined with colistin can be used for polymyxin B.Materials and Methods. A total of 268 strains of Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumoniae), Pseudomonas aeruginosa (P. aeruginosa), and Acinetobacter baumannii (A. baumannii) isolates were included in the study. The reference broth microdilution and the results were evaluated according to CLSI recommendations.Results. Of the 268 strains, 40.7% were found to be colistin resistant and 34% polymyxin B resistant. Essential agreement (EA) and categorical agreement (CA) ratios for all isolates were found to be 81.3% and 85.1%, respectively. Identical MIC values were found for colistin and polymyxin B in 104 isolates (38.8%), and this rate was 31.1% and 48.7% in Enterobacterales and non-fermentative bacteria, respectively. Colistin resistance rates were 25.4% for E. coli, 67.4% for K. pneumoniae, 23.8% for P. aeruginosa, and 31.5% for A. baumannii. Polymyxin B resistance rates were 18.6% for E. coli, 64.1% for K. pneumoniae, 19% for P. aeruginosa, and 16.7% for A. baumannii.Conclusion. According to the results of the study, it was concluded that the MIC values of colistin and polymyxin B antibiotics in all bacteria were not compatible with each other. Colistin susceptibility results cannot be used for treatment with polymyxin B, despite CLSI's recommendation.