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

Abstract 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.