Molecular Typing And Virulence Characteristic Of Staphylococcus Aureus Isolated From Raw Milk

Staphylococcus aureus threatens the safety of raw milk. This paper studied the molecular typing, virulence characteristic, and chlorhexidine and antimicrobial resistance of Staphylococcus aureus isolated from raw milk to evaluate the safety and quality of raw milk and prevent the potential risk. The main conclusions are showed as follows:1. Identification of Staphylococcus aureus isolated from raw milkA total of 51 Staphylococcus aureus isolates were collected by biochemistry experiments from 125 samples of raw milk. Nine MRSA isolates were detected through screening mec A gene.2. Antibiotic susceptibility result of Staphylococcus aureus isolated from raw milkAccording to antibiotic susceptibility result, resistant rates of the 51 Staphylococcus aureus isolates to erythromycin, clindamycin, tetracycline and ciprofloxacin were 52.9%, 33.3%, 37.3% and 31.4%, respectively. In addition, the resistance rates of other antibiotics ranged from 3.9-19.6%. None of the Staphylococcus aureus isolate from raw milk was resistant to teicoplanin, Linezolid and vancomycin.3. Spa typing of Staphylococcus aureus isolated from raw milkThe 51 Staphylococcus aureus isolates were characterized by spa typing. There were 10 spa types including t518, t519, t521, t2875, t377, t127, t189, t114, t437 and t2460. In MSSA isolates, t518(17 isolates), t519(10 isolates) and t521(7 isolates) were predominant. Among MRSA isolates, t114(6 isolates) was predominant. In addition, two spa types(t114 and t437) were human-derived types, which means that raw milk was polluted by handlers during producing, processing and transportation.4. SCCmec typing and PFGE typing of staphylococcus aureus isolated from raw milkThe 9 MRSA isolates were typed by SCCmec typing and PFGE typing. The SCCmec typing results revealed that t114 and t437, and t2460 were typed as SCCmec III and SCCmecIV, respectively. The SCCmec III was predominant in MRSA isolates from raw milk. These 9 isolates were also classified into 5 PFGE clusters including a cluster(3 isolates, 3 PFGE types), the other cluster(3 isolates, 2 PFGE types) and 3 singletons, which means that PFGE types of MRSA isolates isolated from raw milk were varied.5. Virulence genes distribution of Staphylococcus aureus isolated from raw milkThe 51 Staphylococcus aureus isolates were screened for the presence of virulence genes by PCR. The result showed that sea(15.7%), seb(25.5%) and seh(17.6%) were dominant virulence genes. The sec, seg, sei, tsst and etb genes were also found in some isolates.6. Efflux-pump-gene distribution and chlorhexidine MICs of Staphylococcus aureus isolated from raw milkOnly 1 Staphylococcus aureus isolate harbored qacA/B gene, the chlorhexidine MIC of which was 4 mg/L. None of the qacG, qacH, qacJ and smr gene was found in 51 staphylococcus aureus isolates. The chlorhexidine MICs of MSSA isolates were 8 mg/L(4 isolates), 4 mg/L(30 isolates), 2 mg/L(5 isolates) and 1 mg/L(3 isolates). In addition, chlorhexidine MICs of MRSA isolates were 8 mg/L(2 isolates), 4 mg/L(5 isolates), 2 mg/L(1 isolates) and 0.5 mg/L(1 isolates). The result showed that the detection of disinfectant efflux-pump-gene was low, and the dominant chlorhexidine MIC of 51 Staphylococcus aureus isolates was 4 mg/L.