Prevalence and Molecular Characterization of mcr-1 -Positive Salmonella Strains Recovered from Clinical Specimens in China

The recently discovered colistin resistance element,mcr-1 , adds to the list of antimicrobial resistance genes that rapidly erode the antimicrobial efficacy of not only the commonly used antibiotics but also the last-line agents of carbapenems and colistin. This study investigated the prevalence of the mobile colistin resistance determinantmcr-1 inSalmonella strains recovered from clinical settings in China and the transmission potential ofmcr-1 -bearing mobile elements harbored by such isolates. Themcr-1 gene was recoverable in 1.4% of clinical isolates tested, with the majority of them belonging toSalmonella enterica serotype Typhimurium. These isolates exhibited diverse pulsed-field gel electrophoresis (PFGE) profiles and high resistance to antibiotics other than colistin and particularly to cephalosporins. Plasmid analysis showed thatmcr-1 was carried on a variety of plasmids with sizes ranging from ∼30 to ∼250 kb, among which there were conjugative plasmids of ∼30 kb, ∼60 kb, and ∼250 kb and nonconjugative plasmids of ∼140 kb, ∼180 kb, and ∼240 kb. Sequencing of representativemcr-1 -carrying plasmids revealed that all conjugative plasmids belonged to the IncX4, IncI2, and IncHI2 types and were highly similar to the corresponding types of plasmids reported previously. Nonconjugative plasmids all belonged to the IncHI2 type, and the nontransferability of these plasmids was attributed to the loss of a region carrying partial or completetra genes. Our data revealed that, similar to the situation inEscherichia coli ,mcr-1 transmission inSalmonella was accelerated by various plasmids, suggesting that transmission ofmcr-1 -carrying plasmids between different species ofEnterobacteriaceae may be a common event.