| Staphylococcus aureus is a common cause of dairy cow mastitis,causing a huge economic lost on cattle ranches and potentially harmful to public health.Since 1961,the first S.aureus carrying SCCmec genomic island(Methicillin-Resistant Staphylococcus aureus,MRSA)was discovered by Jevons in Britain.MRSA is resistant to all ?-lactam antibiotics and its multidrug resistance has seriously affected the treatment of animals and public health security.It has become a global pathogenic microorganism since 1980s and reported worldwide.On one hand,the existence of mec gene plays an important role in drug resistance of S.aureus.On the other hand,from the previous studies we found that the thermostable nuclease in S.aureus also has a potential influence in the resistant ability.This has aggravated the constraint condition of drug use.By multilocus sequence typing and drug susceptibility test,we aimed at revealing the relationship between molecular epidemiology and drug-resistant spectrum of mecA mediated MRSA.What's even more,we also wanted to disclose the potential functions of nuc gene in antibiotic resistance.These will provide theoretical basis for the epidemiological study and drug resistance mechanism of MRSA in Jiangsu area,and have certain guiding significance for the rational application of clinical antibiotics.The main research contents and results include the following three parts:1.Separation and identification of Staphylococcus isolated from cow mastitis and analysis of drug resistance.A total of 613 CMT-positive samples were collected from December 2015 to September 2017 in Jiangsu province.877 strains of pathogenic bacteria were isolated,Staphylococci were the most commonly isolated bacterial group(n = 392,44.70%of the total bacterial isolations),followed by Streptococcus(19.61%)and Bacillus or others(n=313,35.69%).And then all Staphylococci were identified by mass spectrum detection system of MALDL Biotyper,the results showed that S.sciuri occupy the largest proportion which is 36.03%and the S.aureus was second,accounting for 15.81%.The rest of them were S.xylosus,S.epidermidis and S.arlettae,which made up 12.13%,8.09%and 5.15%separately.After that,we detected the drug resistance of S.aureus to common antibiotics and vancomycin by disk diffusion test and minimal inhibitory concentration(MIC),which are both according to the description of CLSI 2013.The results showed that the resistance rate of S.aureus to penicillin G was the highest,80.92%.Followed by norfloxacin and ciprofloxacin,were 36.64%and 33.59%respectively.We have not yet found the minocycline drug-resistant strains.Almost all Staphylococci are sensitive to vancomycin except 2 strains of Coagulase-Negative taphylococcusare intermediate to it.2.Separation and identification of MRSAand analysis of different ST-type resistance.In this part,114 MRSA and 25 MRCNS were detected from 469 Staphylococcus genomes by triple PCR.MRSA is highly resistant to drug resistance of SCCmec,which has great harm to public health and food safety.In order to understand the epidemiological characteristics and advantages type strains resistant situation of MRSA isolated from cows in Jiangsu region,we used multilocus sequence typing test to classify MRSA.Eventually,we received four ST types,type of ST630(99 strains)was the most advantage,the rest are respectively ST71(2 strains),ST479(2 strains)and ST770(1 strain).ST630 was widely distributed in 2015-2017 and was the absolute dominant and stable type of MRSA isolated from cows in this region.By analysis of drug susceptibility results,ST630 showed higher resistance to the quinolones(norfloxacin,ciprofloxacin,levofloxacin)when compared with other three types.Besides,it also showed a strong resistance to co-trimoxazole,feftazidime and spectinomycin.3.Exploration of the potential infection of nuc gene to the drug resistance in S.aureusCarrying SCCmec genomic island can enhance its ability of drug resistance in S.aureus.Additionally,the unique thermostable nuclease of S.aureus also has a potential role in the resistant ability.Based on the analysis of preliminary results,we found that the resistance rate of S.aureus to antibiotic affecting nucleic acid metabolism was higher than CNS.Therefore,the absence of nuc gene in MRSA was built to compare the changes with wild-type and the potential infection of nuc gene to the drug resistance in S.aureus might appear.The results showed that the resistance of the mutant strain and the wild strain were basically the same,but there was a certain weakening. |
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