Evaluation of phenotypic methods for detection of polymyxin B-resistant bacteria.

Determination of sensitivity to polymyxins has always been a challenge, especially in clinical laboratory routines. This study evaluated two rapid, simple, and inexpensive phenotypic methods to test polymyxin B (PMB) susceptibility in Enterobacterales and non-fermenting Gram-negative bacilli. One hundred isolates were used in the tests. The isolates were collected in three hospitals in southern and southeastern Brazil from 1995 to 2019. We compared broth microdilution (reference method) with the broth disk elution test and modified drop test, using polymyxin B -disk or PMB -powder in 2 concentrations (12 and 16 µg/ml). For the broth disk elution and modified drop test with the concentration of 12 µg/ml, categorical agreement values exceeded 90%. The modified drop test with a concentration of 12 µg/ml and broth disk elution may be excellent for initial screening of polymyxin-resistance in laboratory routines. Moreover, these methods are simple and use inexpensive supplies, and may optimize therapeutic decisions.