Polymyxin NP elution : a rapid and accurate methodology to determine susceptibility to polymyxins among Enterobacterales

Abstract

Fast and accurate detection of polymyxins resistance has become necessary as they remain one of the few last resources to treat infections caused by Carbapenem-resistant Enterobacterales in many regions. We described NP macroelution and established the miniaturized version, NP microelution, aiming to detect polymyxins resistance quickly, accurately and at low cost among Enterobacterales. The methodologies consist of exposing bacterial populations of 10⁸ CFU/mL in NP solution where polymyxin B disks were previously eluted, reaching concentrations of 2μg/mL for macro and 3μg/mL for NP microelution. Eighty-four Enterobacterales isolates were evaluated, 39 (46.4%) resistant to polymyxin B. When compared to broth microdilution (BMD), the NP macroelution obtained 2.4% major error (ME), with specificity of 95.6%, while its miniaturized version presented a slightly lower a ME (1.2%) and higher specificity (97.8%). Both methodologies presented sensitivity of 100.0%, and needed 3 hours incubation to identify over 90% of truly resistant isolates. NP macro and microelution proved to be excellent alternatives to determine polymyxin B susceptibility in routine of microbiology laboratories, presenting low cost, being easy to perform, and demanding short incubation time.