In the last years, the reports of polymyxins resistance have increased considerably, so efficient, reliable, fast and easily interpreted testing to determine polymyxins susceptibility is necessary. The present study aimed to develop and evaluate the performance of a colorimetric microelution method for detecting polymyxin B resistance in clinical isolates of Gram-negative bacilli. The tests were performed with 11 clinical isolates, including 8 E. coli and 3 K. pneumoniae. The colorimetric polymyxin B microelution method was an adaptation of the colistin broth disk elution test, adding resazurin at the end of the process. The results were interpreted using the breakpoints of the BrCAST. When evaluating the microelution results with and without resazurin, 7 isolates presented categorical agreement and 9 and 10 isolates, respectively, presented essential agreement with the reference method. In conclusion, both polymyxin B microelution test and the colorimetric polymyxin B microelution test were easy to perform, requiring materials commonly found in routine microbiology laboratories. Furthermore, a method involving polymyxin B, which is the polymyxin used in Brazil, encourages the study of new susceptibility tests against polymyxins.
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