Detection of Plasmid-Mediated Resistance against Colistin in Multi-Drug-Resistant Gram-Negative Bacilli Isolated from a Tertiary Hospital

The aim of this study was to determine the prevalence of plasmid-mediated colistin resistance <i>mcr</i>-1 to <i>mcr</i>-5 genes among colistin and multi-drug-resistant Gram-negative bacilli strains isolated from patients in a tertiary hospital in Toluca, Mexico. The presence of <i>mcr</i> genes among the 241 strains collected was assessed by PCR. In the case of <i>mcr</i>-carrying <i>E. coli</i>, further PCR tests were performed to determine the presence of <i>bla</i>_CTX-M and whether the strains belonged to the O25b-ST131 clone. Conjugation experiments were also carried out to assess the horizontal transmission of colistin resistance. A total of twelve strains (5.0%), of which four were <i>E. coli</i>; four were <i>P. aeruginosa</i>; three were <i>K. pneumoniae,</i> and one <i>E. cloacae</i>, were found to be resistant to colistin. Of these strains, two <i>E. coli</i> isolates were found to carry <i>mcr</i>-1, and Southern blot hybridization demonstrated its presence on an approximately 60 kb plasmid. Both <i>mcr</i>-1-carrying <i>E. coli</i> strains were found to co-express <i>bla</i>_CTX-M, belong to the O25b-ST131 clone, and horizontally transmit their colistin resistance. The results of this study confirm the presence of plasmid-mediated colistin resistance in hospitalized patients in Mexico and demonstrated that the multi-drug-resistant O25b-ST131 <i>E. coli</i> clone can acquire <i>mcr</i> genes and transmit such resistance traits to other bacteria.