Comparison of the Superpolymyxin™ and CHROMID Colistin R Screening Media for the Detection of Colistin-Resistant Enterobacteriaceae from Spiked Rectal Swabs

The dissemination of carbapenemase-producing Enterobacteriaceae (CPE), has led to the increased use of colistin, which resulted in the emergence of colistin-resistant Enterobacteriaceae worldwide. One of the most threatening scenarios is the dissemination of colistin-resistance in CPE, particularly the plasmid-encoded resistance MCR. Thus, it becomes now mandatory to possess reliable media to screen for colistin-resistant Gram negative isolates, especially Enterobacteriaceae. In this study we evaluated the performances of the Superpolymyxin™ medium (ELITechGroup) and the CHROMID® Colistin R (bioMérieux) to screen for colistin-resistant Enterobacteriaceae from spiked rectal swabs.

Stools were spiked with a total of 94 enterobacterial isolates (Escherichia coli, Klebsiella pneumoniae, Salmonella enterica, Enterobacter cloacae), including 53 colistin-resistant isolates. ESwabs™ (Copan Diagnostics) were then inoculated with those spiked fecal suspensions and proceed as recommended by both manufacturers. The sensitivity of detection colistin-resistant Enterobacteriaceae were of 86.8 % [95 % confidence interval (CI95) 74.0 – 94.0] using both the Superpolymyxin™ medium and the CHROMID® Colistin R plates. Surprisingly, the isolates that were not detected were not the same for both media. The specificities were high for both media, at 97.9% [CI95 = 87.3% – 99.9%] for Superpolymyxin™ medium and 100% [CI95 = 90.4% – 100%] for the CHROMID® Colistin R medium. Both commercially-available media, CHROMID® Colistin R and Superpolymyxin™, provide a useful tool to screen for colistin-resistant Enterobacteriaceae from patient samples (rectal swabs) regardless of the level and mechanism of colistin resistance.