Abstract

Resistance to polymyxin, mediated by chromosomal mutations and plasmid-borne mcr genes, is increasingly being reported not only in clinical bacteria but also in animals, farms, foods, and the environment. In this study, we evaluated a test capable of detecting resistance to polymyxins during antimicrobial susceptibility testing by the agar dilution test. We evaluated 521 consecutive isolates from A. baumannii, P. aeruginosa and Enterobacteriaceae and 57 polymyxin-resistant K. pneumoniae. We performed tests by the agar dilution method for colistin and polymyxin B (colispot and polyspot, respectively), the Broth Micro Dilution (BMD) method, and the VITEK ® 2 automated system for colistin. The colispot and polyspot tests were evaluated in concentrations of 2.0, 4.0 and 8.0 μg/mL. Considering BMD as a reference method, 420 isolates were shown to be colistin sensitive, 158 isolates were colistin resistant, 423 isolates were sensitive to polymyxin B and 155 isolates were resistant to polymyxin B. The AUC of the ROC curve showed better performance for 4.0 μg/mL, with values of 0.9671 and 0.9568 for colispot and polyspot, respectively. The Kappa index was 0.9305 for colispot and 0.9079 for polyspot. All isolates analyzed by VITEK ® 2 exhibited ROC curves with AUC values of 0.9541 and a Kappa index of 0.9049. Colispot and polyspot at a concentration of 4.0 μg/mL had higher results than the VITEK ® 2 system. The results showed that colispot and polyspot can be used with confidence in the practice of clinical laboratories.

Keywords: Polyspot; Colispot; Polymyxins; Resistance; Broth micro dilution; Detection method