An alternative disk diffusion test in broth and macrodilution method for colistin susceptibility in Enterobacteriales.

Colistin and polymyxin B are old drugs that have been reintroduced to treat Gram-negative infections lacking other treatment options; however, colistin resistance have been reported. To know the correct susceptibility pattern is mandatory for multidrug resistant bacteria. Broth microdilution method is the gold standard to evaluate colistin and polymyxin B susceptibility; nevertheless, it is time consuming and needs expertise to be performed. Disk diffusion method on Müeller-Hinton agar is no longer recommended to evaluate polymyxins susceptibility. In this study we evaluated two methods (disk diffusion in broth and broth macrodilution) as alternative options to identify polymyxin resistance in an easy way. A total of 536 Enterobacteriales isolates were assessed for colistin susceptibility. All non-wild type Enterobacteriales (41) were chosen and 31 wild type bacteria were randomly selected, were used to perform disk diffusion tests in broth and for broth macrodilution tests. We found 100% of concordance between both tested methods and broth microdilution. In conclusion, these two methods are reliable and easier options that complement as initial screening susceptibility for colistin in Enterobacteriales in microbiology laboratories lacking personnel and infrastructure to perform broth microdilution method.