Distribution And Molecular Characteristics Of Mcr-1 Gene In Enterobacteriaceae From Aquatic Products

The emergence of mcr-1 gene has brought a new viewpoint to explain the production of colistin resistance.As a representative drug resistance gene mediated by plasmids,there has been a wave of research on it all over the world.Many researches have shown that mcr-1gene is prevalent in a wide range,involving multiple host sources,and there are co-translocations of drug resistance genes in various plasmids.However,there are few reports about mcr-1 gene in aquatic products.In order to better study the epidemic distribution and molecular characteristics of mcr-1 gene.we investigated the prevalence of mcr-like gene in aquatic products in Guangzhou.To provide a theoretical basis for the transmission of antibiotic resistance genes in water environment.In this study,aquatic samples were collected from 123 freshwater fish samples from 9markets,34 crocodile intestinal tract samples and 43 fresh oyster samples,12 water samples,and 12 mud samples from the ocean.7.32%（9/123）fish meat samples and 11.38%（14/123）intestinal tract samples from freshwater fish,44.18%（15/34）crocodile intestine tract samples were mcr-1 positive,The total positive rate of mcr-1 gene of aquatic was 15.18%（34/224）.All mcr-1 positive strains were identified as Escherichia coli.The sensitivity of mcr-1positive isolates was determined by to antibiotics using agar dilution method.The results showed that the resistance rates to tetracycline,sulfamethoxazole/trimethoprim,gentamycin,and streptomycin were 100%,86.84%,84.21%,and 84.21%,respectively.The resistance rate of Nalidixic acid and cefotaxime and ciprofloxacin were 78.95%,followed by 76.32%and71.05%of florfenicol and olaquindox,while they were all sensitive to amikacin and meropenem.The mcr-1 positive strains of freshwater fish from different farmers’markets can be divided into fourteen PFGE clusters,but the same cluster may incorporate isolates from different markets,ST10 was the most popular sequence type,followed by ST1114,ST3937and ST211.The mcr-1 positive strains isolated from the intestines tract of crocodile can be divided into three PFGE clusters.There is little difference in the genetic background of these strains,and 71.42%of the strains are ST117 sequence types.12 mcr-1 positive isolates were successfully transferred.S1-PFGE confirmed that the size of the plasmid carrying the mcr-1gene was between 54.7 kb and 250 kb.Three non-conjugative mcr-1 positive strains were randomly selected for whole genome sequencing,showing that the mcr-1 was integrated in IncHI2 plasmid（n=1）,IncI2 plasmid（n=1）or bacterial chromosome with two copies（n=1）.In addition to mcr-1,a variety of additional antibiotic resistance genes,such as qnrS,floR,oqxAB,aac（6’）-Ib-cr,bla _CTX-M-1GTX-M-1G and fosA3.These data suggest that multiple drug resistance phenomenon and clonal spread of mcr-1 positive isolates exist in freshwater fish in geographical different markets in local area.Aquatic products are an important antibiotic resistance reservoir and it highlights the possible risk regarding food safety.