Rapid colorimetric polymyxin B microelution directly from positive blood bottles: because patients with serious infections should not have to wait for results of culture-based methodologies.

Abstract

To evaluate the performance of the rapid colorimetric polymyxin B microelution (RCPEm) in determining polymyxin B resistance directly from Enterobacterales-positive blood cultures. A set volume of positive blood culture bottles (diluted 1:10) was inoculated into a glucose-broth-phenol red solution (NP solution), where a polymyxin B disk was previously eluted (final concentration of 3µg/mL). Test was read each 1h for up to 4h. Color change from red/orange to yellow indicated resistant isolates. Results were compared to the reference method, broth microdilution (BMD), performed from colonies grown on solid media from the same blood culture bottle. One hundred fifty-two Enterobacterales-positive blood cultures were evaluated, 22.4% (34/152) of them resistant to polymyxin B (including 6.6% with borderline MICs). When performing directly from positive blood cultures (RCPEm-BC), specificity and sensitivity were 99.1% and 94.1%, respectively. Of note, 79.4% (27/34) of truly resistant isolates required 3h of incubation, compared to the 18 ± 2h incubation that microtiter plates of BMD demand before reading can be performed. RCPEm directly from blood cultures has great potential to be part of the routine of clinical microbiology laboratories to establish polymyxin B susceptibility, impacting outcome of patients with bloodstream infections caused by carbapenem-resistant Enterobacterales.