| Staphylococcus aureus(S.aureus)is one of the most prevalent food-borne pathogens that is prone to food poisoning and infection among people.Since the widespread use of antibiotics in hospitals and livestock,bacteria have become increasingly resistant,with constantly methicillin-resistant Staphylococcus aureus(MRSA)being detected.The increasingly upward trend of bacterial resistance not only has an adverse impact on the prevention and treatment of human,but also aggravates the growth and spread of drug-resistant pathogens.In the 1990 s,MRSA strains had been isolated from healthy people in Britain,Canada and other countries.At present,most of the control measures for drug-resistant pathogens in various countries are to optimize the use of antibiotics,establish an effective resistance monitoring system,and encourage the development of new antibiotics and vaccines.Few studies have focused on the whole(environment-food-population)system.This study provides a basis for the prevention and treatment of drug-resistant strains by studying the toxin genes,drug resistance,molecular typing and horizontal transfer of drug-resistant genes of MRSA strains from different sources.The main results are as follows:(1)A total of 76 mec A-positive strains were detected in 758 strains of S.aureus which were collected from a certain region in the past 6 years,with a detection rate of 10.0%.Among them,9 strains were isolated from food,22 from healthy people or hospital population,36 from hospital environment,and 9 from food poisoning samples.(2)All MRSA strains show multiple drug resistance,76(100.0%)could resist AMP and PEN,majorities of isolates could resist T/S(94.7%),FOX(90.8%)and ERY(77.6%),A/C(69.7%),CIP(59.2%),TET and OXA(48.7% for each),while only 34.2% and 25.00%strains show resistance to FOP and GEN,respectively.On the contrary,all MRSA strains were sensitive to vancomycin.sources of strains is closely related to antimicrobial resistance profiles,which means that MRSA strains isolated from the environment and human sources are significantly more resistant than food and food poisoning strains.(3)In this study,there were 33 OS-MRSA strains(mec A positive-oxacillin sensitive-cefoxitin resistant)isolated,with a high detection rate of 43.4%.However,these OS-MRSA strains were sensitive to the majority of the tested antibiotics.In addition to ERY and T/S antibiotics,the OS-MRSA strains were not almost resistant to non--lactam antibiotics.(4)All MRSA strains were detected by PCR for 27 toxin genes,the detection rate of hla and hld gene were both 100.0%,followed by pvl(92.1%),hlb(46.1%),seg(43.4%),sem(32.9%),seo(36.8%),sec and sei(35.5% for each),tst(34.2%),sen(32.9%),sel(22.4%),seb(13.2%),sea,see and seq(11.8% for each),seh(9.2%),sep(7.9%),sek(6.6%),sed(5.3%),ser(3.9%),sej and hlg(2.6% for each).In short,among 27 kinds of toxin genes,egc gene cluster(seg,sei,sem,sen,seo,and/or seu))gene,hemolysin gene(hla,hlb,hld and hlg)and other toxins(tst)and pvl genes were more common,the detection rate 24.8%(133/537),35.2%(189/537)and 17.7%(95/537)respectively,and the complete carry egc gene cluster strains accounted for 11.8%(9/76).(5)Among them,the environmental and human MRSA gene profiles were mainly concentrated in more than 6 genes,and the main gene profiles were sec-seg-sei-sel-sem-senseo-tst-hla-hld-pvl and sed-seg-sei-sej-sem-sen-seo-ser-hla-hld-pvl.The MRSA gene profiles of food and food toxin mainly concentrates under 6 genes.The results indicated that environmental and human MRSA strains were more likely to acquire toxin genes.(6)A total of 20 spa types,7 ST types and 4 SCCCmec types were detected in all MRSA strains,,except for some strains were not determined.MRSA strains from different sources show different molecular proflies: environmental source of main clone strains for ST5-MRSA-Ⅱ-t002(44.4%),people source of main clone strains for ST5-MRSA-Ⅱ-t002(27.3%)and ST59-MRSA-Ⅲ-t437(22.7%),foods source of main clone strains for ST59-MRSA-Ⅳa-t437(33.3%)and food poisoning source of main clone strains for ST398-MRSA-IVa-t034(44.4%).(7)The dominant clone ST5-MRSA-Ⅱ-t002 and ST59-MRSA-t437 were selected as the research objects,and the transformation,conjugation and transduction tests were used to study the transmission path of erythromycin resistance genes.The results showed that erythromycin resistance genes could be transfered by transformation and phage transduction,but the conjugation test was not successful,possibly due to the low conjugation frequency. |
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