Investigation On Antimicrobial Resistance And Biofilm Formation Of Foodborne Methicillin-resistant Staphylococcus Aureus

Staphylococcus aureus (S. aureus) is a common foodborne pathogenic microorganism,which can cause food poisoning. In recent years, methicillin-resistant Staphylococcus aureus(MRSA) strains have been identified in various foods and food animals due to the abuse ofantibiotic in the farming. Besides, the MRSA isolates have the ability of adherence andbiofilm-formation on various biomaterials, which leads to contamination, especially repeatedinfection. It is necessary for us to investigate antibiotic resistance and biofilm-formation ofMRSA isolates from food and food animals.In this study, antibiotic resistance and biofilm-formation of34MRSA isolates wasinvestigated, icluding12isolates from pig nasal,4isolates from raw milk,6isolates fromchicken,6isolates from dumplings. We used agr dilution method to study the antibioticresistance, used CRA (Congo Red Agar）and in vitro microtiter plate assay to test andquantify biofilm formation. We then investigated the presence of several virulencedeterminants of antibiotic resistance and biofilm formation by PCR assay. The main resultsare as follows.(1) The antibiotic resistance of MRSA isolates in present study was serious. Theresistance to SXT and ERY was predominant, followed by CEFOX, GEN, AMP, TET andothers. All isolates were sensitive to VAN. In addition, the different areas and resources ofMRSA exhibited different antibiotic resistance. The isolates from ready-to-eat food andquick-frozen dumplings showed weak antibiotic resistance compared with others.(2) Fifteen antibiotic profiles were detected among MRSA isolates.AMP-CEFOX-OXA-CHLO-TET-AMC-GEN-SXT-ERY was the most commonantimicrobial resistant profile.(3) For antibiotic resistance genes detection, blaZ94.1%（32/34） was the highest.Genotypic and phenotypic antibiotic resistance of MRSA differed significantly (P＜0.05)among different antibiotics. (4) For resistance of cefoxitin was higher than oxacillin. The PBP2a protein encoded bymecA and β-lactamase encoded by blaZ were the main cause of resistance to β-lactam drugsin MRSA.(5) The CRA analysis was conducted for detecting the biofilm formation of MRSAisolates. The results showed that all isolates except one from ready-to-eat food showedbiofilm positive. All isolates have the ability of biofilm formation at various degrees bymicrotiter plate method. Weakly biofilm-producing isoaltes were predominant, followed bymoderately and strongly biofilm-producing isolates. All MRSA isolates showed that percentresistance to erythromycin, cefoxitin, tetracycline, chloromycetin, amoxicillin/clavulanicacid and ampicillin differed significantly (P＜0.05) among different biofilm-producingisolates. These data suggest that there is a significant association of biofilms with multipledrug resistance in MRSA isolates from food animals and retail foods.(6) The biofilm-producing ability and genes related with biofilm formation differedsignificantly (P＜0.05) among different types of food isolates. Not all isolates were positivefor icaADBC gene, including50%positive for icaBC,32.4%for icaAD. We found that theprevalences of agr，sar and sigB were17.6%，5.9%，2.9%respectively. The prevalence ofclfA and eno (97.1%) was highest among MSCRAMMs genes, followed by clfB (91.2%), fib(82.3%), ebpS (58.8%), icaBC (50%), fnbB (35.3%), icaAD (32.4%), cna (14.7%), fnbA(8.8%), bap (2.9%), and bbp (2.9%).