Isolation Method Optimization And Genetic Analysis Of Livestock-associated Methicillin Resistant Staphylococcus Aureus

Many researches showed that livestock-associated methicillin resistant Staphylococcus aureus (LA-MRSA) has high isolation rate in pigs and can transmit to community through meat and workers directly contact with pigs. This study optimized the isolation method of the LA-MRSA and did genetic analysis for the isolations from samples collected in Beijing, Shanxi and Inner Mongolia in China. With the whole information got in this research, the epidemiology of LA-MRSA in China was studied.Methods:This study screened NaCl, aztreonam, polymyxin B, sodium azide, and nalidixic acid using MRSA and non-MRSA strains from different sources to determine the best inhibitive substances and their concentration used in the selective broth. The study collected minced pork samples and nose swab samples from pigs and workers directly contacted with pigs and did MRSA isolation for these samples. The MRSA isolations from community people were collected. And spa and MLST typing were performed for all the MRSA strains we got.Results:The selective broth with6.5%NaCl inhibited the growth of some MRSA strains. Improving the high salt concentration selective broth with the second enrichment using selective broth with4%NaCl、100μg/mL aztreonam、10μg/mL polymyxin B、100μg/mL nalidixic acid and4μg/mL cefoxitin could get isolations which were sensitive to high concentration salt.In Beijing, we got33(22.9%) MRSA isolations from144minced pork samples collected from supermarkets, in which13(39.4%) ST9/t899MRSA were determined;1MRSA isolation from pig and1MRSA isolation from worker directly contact with pigs and all of them were ST9/t899MRSA. In Hohhot, we didn’t get MRSA positve samples from34minced pork samples and96swabs from workers directly contact with pigs. In Shanxi, the MRSA isolation rates of pig swab samples, worker swab samples and minced pork samples were2.8%(n=216),33.3%(n=60), and7.5%(n=146), respectively. And in the typed isolations, the rates of the ST9/t899MRSA were100%(4/4),50%(8/16) and27.3%(3/11), respectively. And81MRSA isolations were collected from community, in which73isolations were typed and24(32.9%) were ST9/t899MRSA. All the ST9/t899MRSA were resistant to cefoxitin and clidamycin. And for the erythrocin, tetracycline and ciprofloxacin, the resistance rates were93.0%、97.7%and95.3%, respectively.Conclusions:The two times enrichment method could get MRSA isolations which were sensitive to high concentration salt and higher MRSA isolation rate combined with the high salt concentration enrichment. The main LA-MRSA was ST9/t899in China, and it was multi-drug resisitance. All the ST9/t899MRSA were resistant to cefoxitin and clidamycin. And for the erythrocin, tetracycline and ciprofloxacin, the resistance rate were93.0%、97.7%and95.3%, respectively. ST9/t899MRSA isolations got from pigs, workers directly contacted with pigs, minced pork samples and community people in Hanzhong city indicated that LA-MRSA had transmit to community throuth minced pock and workers directly contact with pigs.