Occurrence and Characteristics of Mobile Colistin Resistance ( mcr ) Gene-Containing Isolates from the Environment: A Review

The emergence and spread of mobile colistin (COL) resistance ( mcr ) genes jeopardize the efficacy of COL, a last resort antibiotic for treating deadly infections. COL has been used in livestock for decades globally. Bacteria have mobilized mcr genes ( mcr -1 to mcr -9). Mcr-gene-containing bacteria (MGCB) have disseminated by horizontal/lateral transfer into diverse ecosystems, including aquatic, soil, botanical, wildlife, animal environment, and public places. The mcr -1, mcr -2, mcr -3, mcr -5, mcr -7, and mcr -8 have been detected in isolates from and/or directly in environmental samples. These genes are harboured by Escherichia coli , Enterobacter , Klebsiella , Proteus , Salmonella , Citrobacter , Pseudomonas , Acinetobacter , Kluyvera , Aeromonas , Providencia , and Raulotella isolates. Different conjugative and non-conjugative plasmids form the backbones for mcr in these isolates, but mcr have also been integrated into the chromosome of some strains. Insertion sequences (IS) (especially IS Apl1 ) located upstream or downstream of mcr , class 1–3 integrons, and transposons are other drivers of mcr in the environment. Genes encoding multi-/extensive-drug resistance and virulence are often co-located with mcr on plasmids in environmental isolates. Transmission of mcr to/among environmental strains is clonally unrestricted. Contact with the mcr -containing reservoirs, consumption of contaminated animal-/plant-based foods or water, international animal-/plant-based food trades and travel, are routes for transmission of MGCB.