Antimicrobial Susceptibility,and Molecular Characterization Of Staphylococcus Aureus Isolated From Raw Milk In China

Raw milk is commonly found to contain complex microbial community.Inappropriate or excessive application of antimicrobial agents may lead to the problems of antibioticresidues and antimicrobial resistant strains in raw milk.Antimicrobial resistance not only seriously affected the development of livestock and poultry breeding,but also posed aserious threat to food safety and public health.Staphylococcus aureus(S.aureus)is one of the main pathogens involved in dairy cow mastitis.Monitoring of antibiotic use wouldprove useful to assess the risk of S.aureus in raw milk.Antimicrobial Susceptibility,and Molecular Characterization of Staphylococcus aureus Isolated from Dairy Herds in Northern China.The objective of this work was to investigate the prevalence of S.aureus strains isolated from raw milk in Northern China,and to characterize antimicrobial susceptibility of these strains and their key virulence genes.A total of 195 raw milk samples were collected from 195 dairy farms located in four different cities of Northern China from May to September in 2015.Out of 195 samples,54 samples(27.7%)were detected positive for S.aureus.Among the 54 samples,54 strains of S.aureus were isolated,and 16 strains were identified as methicillin-resistant S.aureus(MRSA).The strains exhibited high percentages of resistance to penicillin G(85.2%),ampicillin(79.6%),and erythromycin(46.3%).Moreover,72%of the strains showed resistance to more than one antimicrobial agent.63%of the penicillin-resistant strains possesses blaZ gene,and 60%of the erythromycin-resistant strains possesses erm(A),erm(B),erm(C),msr(A)or msr(B)genes with eight different gene patterns.All the isolates resistant to gentamicin,kanamycin and oxacillin carried aac6’-aph2",ant(4’)-la and mecA genes,respectively.Two tet(M)positive isolates carried specific genes of Tn916-Tn1545 transposon.The most predominant virulence genes were sec,sea,and pvl,which encoding enterotoxins and Panton-Valentine leukocidin.Thirty-two isolates(59.2%)harbored one or more virulence genes.The majority of S.aureus strains were multidrug resistant and carried multiple virulence genes,which may pose potential risk to public health.Our data indicated that antimicrobial resistance of S.aureus was prevalence in the dairy herds in Northern China,and that usage of antibiotics,especially penicillin G and ampicillin should be in caution for mastitis caused by S.aureus in dairy herds in Northern China.Antimicrobial Susceptibility,and Molecular Characterization of Staphylococcus aureus Isolated from Different raw milks in China.The objective of this work was to investigate the prevalence of S.aureus in different raw milks,including goat,buffalo,camel and yak milk obtained from 5 provinces from China,and to characterize these strains by antimicrobial susceptibility and key virulence genes.Raw milk samples of goat(n=50),buffalo(n=25),camel(n=25),and yak(n=25)were collected from 5 provinces in China in 2016.Out of 125 samples,36(28.8%)strains of S.aureus were isolated,including 16 strains in goat milk,9 strains in buffalo milk,6 strains in camel milk,and 5 strains in yak milk.Among the 36 strains,antimicrobial resistance was most frequently observed to penicillin G(50%),followed by to tetracycline(41.7%),gentamicin(36.1%).Moreover,52.8%of the strains showed multi-drug resistance(MDR).The antimicrobial resistance S.aureus isolates were the most frequently observed in goat milk(31.8%),followed by camel(18%),buffalo(16.2%),and yak(7.69%).Among the 26 resistant strains,61.5%of isolates harbored more than two resistant genes.The antimicrobial resistance genes were the most frequently detected in S.aureus strains from goat milk(26.9%),followed by camel milk(14.1%),buffalo milk(10.24%)and yak milk(7.69%).The most predominant resistance genes were blaZ(50%),aac6’-aph2"(36.1%),and tet(M)(27.8%).The catA,erm(A)and msr(A)genes were not detected in S.aureus isolates.The mecA,ant(6)-la and fexA gene were only detected in S.aureus isolates from goat milks.The tet(M),tet(L),erm(B)and erm(C)genes were only detected in yak,buffalo,camel and buffalo milks respectively,except goat milks.The tet(K)and tet(O)genes were only detected in camel and yak milks respectively.The most predominant toxin genes were sec(8/36,22.2%),followed by sea(3/36,8.3%),seg(3/36,8.3%),tst(3/36,8.3%),which encoding enterotoxins and toxic shock syndrome toxin.The majority of S.aureus strains were multidrug resistant and carried multiple virulence genes,which may pose potential risk to public health.Our finding indicated that the prevalence and antimicrobial resistance of S.aureus was a serious concern in different raw milks in China,especially goat milks.