The emergence and transmission of the mobile colistin resistance gene (mcr-1) threatened the extensive use of polymyxin antimicrobials. Accumulated evidence showed that the banning of colistin additive in livestock feed efficiently reduce mcr-1 prevalence, not only in animals but also in humans and environments. However, our previous study has revealed that a small proportion of Escherichia coli could continually carry chromosomally-encoded mcr-1. The chromosomally-encoded events, indicated the existence of stabilized heritage of mcr-1 and revealed a potential threat in the antimicrobial stewardship interventions, are yet to be investigated. In this study, we systematically investigated the genetic basis of chromosomally-encoded mcr-1 in prevalence and potential mechanisms of lineage, plasmid, insertion sequence, and phage. Our results demonstrated that the emergence of chromosomally-encoded mcr-1 could originate from multiple mechanisms, but mainly derived through the recombination of ISApl1/Tn6330. We reported a specific transmission mechanism, which is a phage-like region without lysogenic components, could associate with the emergence and stabilization of chromosomally-encoded mcr-1. These results highlighted the potential origin and risks of chromosomally-encoded mcr-1, which could be a heritable repository and thrive again when confronted with new selective pressures. To the best of our knowledge, this is the first study to systematically reveal the genomic basis of chromosomally-encoded mcr-1, and report a specific transmission pattern involved in phage-like region. Overall, we demonstrate the origin mechanisms and risks of chromosomally-encoded mcr-1. It highlights the need of public attention on chromosome-encoded mcr-1 to prevent from its reemergence.
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