Colistin resistance screening by 3 μg/ ml colistin agar in Carbapenemase‐producing Enterobacterales

Background In low‐ and middle‐income countries, the use of colistin in therapeutic regimens is common, to treat infections produced for Carbapenemase‐producing Enterobacterales (CPE) due to limited access to the recently discovered‐approved antibiotics. Furthermore, the technical limitations to perform colistin susceptibility tests make it difficult to assess the suitability of this treatment for each patient, as well as to monitor the rates of resistance. In the present study, we describe the use of agar dilution using a unique colistin concentration of 3 μg/ml to discriminate isolates with colistin resistance in CPE obtained from clinical samples. Methods Clinical Laboratory Standards Institute (CLSI) colistin broth microdilution method and dilution agar with a colistin concentration of 3 μg/ml were performed in 168 isolates of CPE obtained from clinical samples in Guayaquil, Ecuador. Broth microdilution was considered our gold standard using CLSI breakpoints as reference (≤2 μg/ml intermediate and ≥4 μg/ml resistant). Categorical agreement was defined as obtaining a reading within the same category with both methodologies. Results Isolates obtained from respiratory samples were the most prevalent (26.19%; n  = 44). Klebsiella pneumoniae was the predominant specie (94.04%; n  = 158). KPC‐like carbapenemase was present in all the isolates, and interestingly, colistin resistance was not mediated by MCR‐1 production. Categorical agreement between both methods resulted in 97.02%. Conclusion We propose the use of dilution agar with a colistin concentration of 3 μg/ml, as a valid method for screening colistin resistance in low‐ and middle‐income countries to monitor resistance and to perform epidemiological studies.