| The discovery and use of penicillin has successfully saved thousands of lives.However,with the irrational use of antibiotics,the problem of resistance to bacteria of animal origin has increased substantially.The development of antibiotics resistance of bacteria of animal origin is much faster than the new drug research and development.Therefore,study on the epidemiology and mechanism of transmission for some important drug resistance genes is one of the important way to control the rapid spread of drug resistance.In recent years,the emergence of multidrug-resistant Streptococcus suis(S.suis)has brought severe challenges to clinical prevention and control of swine streptococcal diseases.The gene optrA is the second transferable resistance gene,which confers cross-resistance to oxazolidinones and phenicols.Oxazolidinones,including linezolid and tedizolid,are recognized as the last-resort antimicrobial agents for the control of clinical infections caused by multidrug-resistant Gram-positive pathogens.Therefore,it has attracted great attention from scholars.Although optr A gene has been reported sporadically in S.suis,there is still no systematic research on its epidemic characteristics and transmission mechanism in S.suis in China.Therefore,carrying out this research provides theoretical support for the clinical prevention and control of the rapid spread of multidrug-resistant S.suis.Healthy pig lung samples were collected from slaughterhouses in Henan,Heilongjiang,Guangdong,Ningxia,Xinjiang,Qinghai,Jiangxi and Inner Mongolia,respectively in China from 2017 to 2019.Isolation and identification of S.suis using THA fresh blood plate,and analysis of genetic relationship between strains by PCR and molecular typing.The minimal inhibitory concentration of bacterial strain against 14 antibiotics was determined by microbroth dilution method.The location of optrA gene was determined by S1-PFGE and Southern blot.The Oxford Nanopore sequencing technology combined with the Illumina sequencing technology were used to perform genome-wide sequencing of representative strains and comparative analysis of the genetic environment of optrA gene.Seven novel optrA mutants were identified by homologous recombination and electric shock transformation.The results showed that 98 strains of S.suis were isolated and identified from lung samples collected from 8 slaughterhouses in different regions of China.The isolation rate of optrA was25.5%(25/98),and the optrA gene was located on the chromosome of S.suis.The results of serotype,PFGE and the MLST showed diversity.Ten new MLST types were found.All isolates displayed high resistant rates to many different classes of antibiotics,such as tetracycline,flufenicol and erythromycin.Whole-genome sequencing showed that IS1216E + optrA+IS1216E structure was found in 8 strains,and circular intermediates were detected by inverse PCR.The optrA gene in some strains is located on the integration and conjunction element(ICE)ICESa2603,and ICESa2603 can be transferred to enterococci across species.Seven new mutants of optrA all are active.The research results suggest that optrA-positive S.suis isolated from healthy pig herds in slaughterhouses in different regions of my country undoubtedly poses a potential threat to food safety and people's health.In summary,the optrA gene can be inserted into the chromosome to form a circular intermediate with the insertion sequence IS1216E,and the gene can also be excised from the chromosome mediated by ICE and then spread horizontally to other bacteria.Therefore,the optrA gene is transmitted horizontally in these ways.This study provides a scientific basis for the prevention and control infections caused by optrA-positive S.suis. |
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