| Polymyxin is regarded as as a last-resort antibiotic in the clinical treatment of infection by multi-drug resistant Gram-negative bacteria,and the resistance of this antibiotic has attracted widespread attention.Since the plasmid mediated colistin resistance gene mcr-1 firstly reported in China in 2015,it has been detected from different sources(human,animal and environment)in Enterobacteriaceae,which seriously threatens the global public health and safety.Therefore,many countries and international organizations have adjusted the use and management policies of colistin in animal.In China,colistin has been banned for the use of animal growth since April 2017.To explore the development of colistin resistance gene mcr-1 and the main plasmids bearing mcr-1 in Escherichia coli from swine,and the impact on the prevalence as well as distribution of antibiotic resistance gene after the implementation of antibiotic use policies,this research investigated the epidemiological characteristics of mcr-1 gene and plasmids carrying mcr-1 in Escherichia coli isolates from swine from 2013 to 2018,thus to provide basis for controlling the rapid spread of the mcr-1 gene and instruction for antibiotics as therapeutic drugs.From 2013 to 2018,a total of 5143 swine fecal samples were isolated and identified3973 Escherichia coli,including 727 strains from a farm in Guangdong province,2075 strains from a slaughterhouse in Guangdong province and 1171 strains from a slaughterhouse in Liaoning province.Agar dilution method was used to detect the antibiotic susceptibility.Results showed that the resistance rates to ampicillin,streptomycin,tetracycline,doxycycline and florfenicol were higher than 90%.In addition,the resistance rate to colistin was 29.78%(n=1183).Moreover,the resistance rates to colistin of Escherichia coli from a farm and a slaughterhouse in Guangdong,and a slaughterhouse in Liaoning were 28.34%,24.77% and 39.54%,respectively.PCR detection of mcr-1 gene,Inc X4,Inc I2 and Inc HI2 plasmids were conducted for3973 Escherichia coli.Results showed that there were 1184 strains(29.80%)carrying mcr-1 gene.The detection rate of mcr-1 increased from 28.21% in 2013 to 47.61% in 2015,and then decreased to 1.69% in 2018.In addition,the detection rates of the Inc X4,Inc I2 and Inc HI2 plasmids were 14.80%(n=588),7.80%(n=310)and 5.29%(n=210),respectively.The detection rate of Inc X4 plasmid increased from 9.91% to 27.15% from2013 to 2015 and then decreased in the following years.The highest detection rate of Inc I2 plasmid was 12.7% in 2016,and then decreased sharply to 1.5% in 2018.The detection rate of Inc HI2 plasmid was fluctuated between 3.76% and 7.13% in six years,and the highest detection rate was 7.13% in 2018.PCR and whole genome sequencing were used to detect the flanking regions of mcr-1and identify the plasmid types carrying the mcr-1.A total of four different genetic environments of mcr-1 were found: mcr-1 without ISApl1(n=634),mcr-1 with ISApl1 in the upstream(n=266),mcr-1 with ISApl1 in the downstream(n=8),and mcr-1 with ISApl1 in the upstream and downstream(n=288).To 757/1184 mcr-1-positive Escherichia coli,mcr-1 gene was located on Inc HI2,Inc I2 and Inc X4 plasmids.In addition,the mcr-1 was integrated in p O111 plasmid(n=8),Inc Y plasmid(n=5),Inc HI1 plasmid(n=1)and bacterial chromosome(n=8).In addition,compared and analyzed the detection rates of Inc X4,Inc I2 and Inc HI2 plasmids carrying mcr-1 in Escherichia coli from 2013 to 2018.It was found that,for mcr-1-Inc X4 and Inc X4 plasmids,the trend of detection rate in Escherichia coli were similar--increasing first and then decreasing;The similar trends were also observed in mcr-1-Inc I2 and Inc I2 plasmids;However,the detection rate of mcr-1-Inc HI2 in Escherichia coli was decreasing,while the detection rate of Inc HI2 was increasing.Microsoft Excel 2016 was used to analyze the correlation of mcr-1 gene and plasmids in Escherichia coli.The results showed that the detection rate of mcr-1 gene showed a strongest relationship with mcr-1-Inc X4 and Inc X4 plasmids.In summary,the Inc X4 plamid is the most popular plasmid type carrying mcr-1 in swine Escherichia coli.The detection rate of mcr-1 gene and the Inc X4,Inc I2 and Inc HI2 plamids bearing mcr-1 all showed a sharp decline starting in 2016,which may attribute to the prohibition on colistin use as feed additive in 2017.It is suggested to reduce the use of antimicrobials and improve the effectiveness of colistin on Enterobacteriaceae to effectively reduce the prevalence of mcr-1 and its plasmid vector.Therefore,we should regulate the use of drugs in animal husbandry and strengthen the monitoring of bacterial resistance to protect human health and ensure the sustainable development of animal husbandry. |
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