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Global spread of mobilized colistin resistance gene (mcr)-carrying Escherichia coli (MCR-EC) poses serious threats to public health. This study aimed to provide insights into different threats posed by two major mcr variants: mcr-1.1 and mcr-3.1.Genetic backgrounds and characteristics of mobile genetic elements carrying mcr-1.1 or mcr-3.1 in 74 MCR-EC isolated from swine farms were analyzed, and comparative genomic analysis was performed with the public sequence database.The mcr-1.1 showed high horizontal transferability (6.30 logCFU/ml). Genetic background of mcr-1.1, including genetic cassette/plasmid, was transferred without insertion sequences (ISs) and/or multi-drug resistance (MDR) and highly shared across strains. The major mcr-1.1-cassette was "mcr-1.1-pap2", mainly encoded in IncI2 and IncX4. Mcr-3.1 exhibited relatively lower conjugation frequency (0.97 logCFU/ml). The mcr-3.1-cassette was flanked by IS26 and was highly variable across strains due to insertion, deletion, or truncation of IS6100, IS4321 or IS5075. Near the mcr-3.1-cassette, MDR regions consisting of antimicrobial/heavy metal resistance genes were identified, which varied across strains. From MCR3-E13 strain, mcr-3.1-carrying IncHI2-fragment was integrated into bacterial chromosome via IS26-mediated co-integration. To the best our knowledge, this was the first study to describe that mcr-3.1-carrying plasmid could be inserted into the bacterial chromosome.Based on high horizontal transferability, mcr-1.1 could play a major role on colistin resistance propagation. On the other hands, mcr-3.1 could be transmitted with MDR together and had dual pathways mediated by plasmid transfer (horizontal transmission) and chromosomal insertion (vertical transmission), enabling it to proliferate stably despite of it's lower horizontal transferability. |
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