Detection of colistin-resistant Gram-negative rods by using the SuperPolymyxin medium

The commercial SuperPolymyxin medium (ELITechGroup, Puteaux, France), currently licensed in Europe, is aimed to screen for either intrinsic or acquired colistin resistance in Gram negatives. It was tested for screening colistin-resistant Gram-negative rods from bacterial cultures of 145 colistin-resistant and 86 colistin-susceptible Gram-negative rods isolates. It exhibited a sensitivity and specificity of 95.2% (138/145) and 95.3% (82/86), respectively. The sensitivity for the detection of the 25 isolates carrying plasmid-mediated colistin resistance genes (mcr-1, mcr-2, mcr-3, and mcr-4) from bacterial cultures was of 100%. It was also evaluated for the detection of colistin-resistant Gram-negative rods from 57 rectal swab samples (41 clinical rectal swab samples and from 16 spiked rectal swab samples supplemented with colistin-resistant enterobacterial isolates carrying mcr genes). The overall sensitivity and specificity were of 100% (33/33) and 90.3% (56/62), respectively. The detection of isolates carrying plasmid-mediated mcr-like genes from rectal swabs was achieved with a lowest detection limit of 103 to 104 CFU/mL. The SuperPolymyxin medium is adapted for screening and detection of colistin-resistant isolates from bacterial cultures and rectal swab samples, regardless of the level and the mechanism of colistin resistance. It might be useful in the context of prevention of outbreaks due to colistin-resistant isolates and for epidemiological surveillance.