| Staphylococcus aureus(S.aureus)is a major zoonotic pathogen,which can cause pneumonia,pericarditis,sepsis and other systemic infectious diseases.As there is no effective vaccine developed to resist against S.aureus infection,treatment with reasonable antibiotics is the main therapy method to prevent and control S.aureus infection.Since the 1930s,with the excessive dependence on antibiotics in the clinical treatment of bacterial infections by medical workers,the increased resistance of S.aureus to antibiotics has become evident and caused the production of methicillin-resistant S.aureus(MRSA).Recent studies have shown that the colonization of MSSA(Methicillin-sensitive S.aureus)and MRSA is not only limited to the nasal cavity and skin,but can also colonize in the intestine;the MRSA can not only spread in a single species,but also transmit between humans and animals;the transmission routes of MRS A have also become more diverse:not only in farms,slaughterhouses,hospitals,communities,but also in retail markets and related surrounding environments.In this study,a total of 4,054 samples were isolated and identified from feces or anal swabs from different animals in farms,swabs from slaughterhouses and their surroundings,and anal swabs from hospital patients.A total of 410 strains of S.aureus were isolated,including 342 strains from 2,932 samples of feces or anal swabs,68 strains from 1,122 swab samples wiped from the slaughter process and the surrounding environment.Antimicrobial resistance tests,minimum inhibitory concentration(MIC)tests,and molecular genetic characteristics studies were performed to reveal the characteristics of these isolates.The collected samples were firstly enriched,purified,and cultured on the selective medium for isolation of S.aureus identified by PCR analysis of the unique nuc(heat-resistant nuclease gene)gene,and the resistance to oxacillin or methicillin antibiotics was further performed to differentiate MRSA from MSSA,which was confirmed by identification of the mecA gene(encoding the penicillin binding protein PBP2).The results showed that there were 342 strains(11.6%)of S.aureus colonized in the intestine,including 66 from pigs,90 from chickens,94 from cattle,6 from wild animals,and 91 from humans.There are 68 isolates of S.aureus detected in the slaughterhouse and its surrounding environment,including 39 from pigs,26 from chickens,1 from cattle,and 2 from wild animals.The disc diffusion susceptibility test and MIC test was performed to analyze the antimicrobial resistance phenotypes of 342 S.aureus strains isolated from the intestine.The resistance rate of these strains to the selected 14 antibiotics in descending order is shown in the following order:99.1%of aztreonam,76.7%of penicillin,50.4%of erythromycin,47.2%of tetracycline,38.1%of ciprofloxacin,36.4%of kanamycin,36.1%of clindamycin,31.2%of gentamicin,29.7%of chloramphenicol,28.2%of cefoxitin,12.8%of sulfamethoxazole,and all of the strains are susceptible to rifampicin,linezolid and nitrofurantoin.The MIC test showed that only 2(3%)pig-derived isolates that were resistant to vancomycin,the remaining strains didn't show MIC>2?g/mL,but most of the strains displayed intermediate drug resistance.The antimicrobial resistance of S.aureus strains isolated from the slaughter house and its surrounding environment showed the following resistance rate:100%of aztreonam,64.7%of Penicillin,63.2 of tetracycline,51.2%of erythromycin,42.6%of clindamycin,41.1%of ciprofloxacin,29.4%of kanamycin,20.6%of chloramphenicol,5.9%of nitrofurantoin,4.4%of cefoxitin,2.9%of sulfamethoxazole,1.5%of gentamicin.All of the strains showed susceptibility to rifampin and linezolid.The MIC test showed that all of the strains were sensitive to vancomycin.Genotype analysis for the prevalence of 16 drug resistance genes[spc,aadD,aph(3')-?,aac(6)-aph(2"),blaZ,mecA,norA,dfrG,tetK,erm(A),erm(B),erm(C),vga(E),msr(A),msr(C),lnu(A)]in human MRSA strains showed that all of the strains are multi-drug resistant,and these strains are resistant to all of the quinolone drugs.The most severely resistant strains are human-derived strains from a certain hospital,belonging to the CC5 family of clonal complexes,and the most common spa type t002.Spa typing was then performed to analyze the genetic characteristics of the S.aureus strains from pigs,chickens,cattle,wild animals,and humans.Ten spa types were identified in 105 pig-derived strains with t011(9.7%)as the predominant spa type.Six spa types were identified in 116 chicken-derived strains with t002(16.1%)as the predominant spa type.Thirteen spa types were identified in 95 bovine strains with t2734(55.3%)as the dominant spa type.Both t377 and t034 was identified as the dominant spa types in 6 strains from 5 species of wild animals.The 91 human-derived strains have the most diverse spa types,containing 40 spa types with t002 as the predominant spa type.MLST analysis showed that 44 identified MRSA strains can be divided into 14 ST types,indicating the genetic diversity of MRSA in the process of evolution.Among them,human MRSA strains take up 77.3%of all the identified ST types,while the ST type of animal-derived MRSA strains is relatively single,accounting for 22.7%.Additionally,the 44 MRSA strains can also be divided into 10 CC types(Clonal complexes),and human MRSA strains showed the most abundant and diverse CC types,of which CC1(18.2%)as the predominant type.The CC types of animal-derived MRSA strains are relatively simple,containing CC398,accounting for 20.5%.In summary,CC5 mainly exists in hospital-related MRSA(HA-MRSA),while CC398 mainly exists in livestock-related MRSA(LA-MRSA)strains,while human-derived strains of CC398(spa t011,t1451)also exist in strains from animals,indicating the potential transmission of MRSA between humans and animals.In addition,CC5 is confirmed as the predominant clonal complexes of human-derived strains in China,and it is also the clone complex with serious multi-drug resistance. |
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