Performance of CHROMID® Colistin R agar, a new chromogenic medium for screening of colistin-resistant Enterobacterales.

Recent emergence of transferable plasmid-borne colistin resistance (mcr genes) raised fear for pan-resistance. We evaluated the performance of a new chromogenic medium [CHROMID® Colistin R agar (COLR)] for the screening of colistin-resistant Enterobacterales. Specificity was evaluated using 89 rectal swabs and 89 stools prospectively collected. COLR sensitivity was evaluated by seeding 59 negative clinical samples artificially contaminated (105 CFU/mL) with 59 colistin-resistant Enterobacterales, including 20 mcr-1-positive strains. Twelve samples with an Enterobacterales with nonintrinsic resistance to colistin were recovered during the specificity study, including one mcr-1-positive Escherichia coli, representing a 6.7% prevalence of colistin resistance in fecal carriage. Overall, specificity was 100.0% [95% CI: 97.8-100.0] and sensitivity yielded 88.1% [95% CI: 77.5-94.1]. False negatives corresponded to 3 Enterobacter spp. (MIC>64 mg/L), 2 Salmonella spp. (MIC = 16 mg/L), 1 E. coli (MIC = 4 mg/L), and 1 K. pneumoniae (MIC = 8 mg/L). COLR appears to be a sensitive and specific chromogenic agar for screening colistin-resistant Enterobacterales, including those carrying mcr-1 gene.