Molecular Epidemiology And Clinical Infection Characteristics Of Methicillin-resistant Staphylococcus Aureus

[Background] Methicillin-resistant Staphylococcus aureus(MRSA)is one of the most prevalent clinical pathogens isolated from hospital settings,and has increasingly identified in community settings.S.aureus can cause a series of diseases,such as skin and soft tissue infection,bacteremia,osteomyelitis,infectious endocarditis and necrotizing pneumonia.Since 1960,MRSA had disseminated in hospital and health care organization worldwide.The high drug resistant rate of MRSA had become an extremely difficult problem for clinicians and threated the health and life of patients.Therefore,investigation of MRSA epidemiology is critical to provide important information for clinicians to prevent and treat MRSA infection.Molecular typing technologies are usually applied to investigate the evolution and epidemiology of MRSA.Currently,the most commonly used molecular typing methods internationally include: spa typing,SCCmec typing,MLST,PFGE and MLVA.S.aureus can produce various toxin,which play an important role in the pathogenisis.About 20% S.aureus strains bear the tst-1 gene,which encodes toxic shock syndrome toxin 1(TSST-1).The structure of TSST-1 is similar to exotoxin and can induce toxic shock syndrome.Panton-Valentine leucocidin(PVL)is cytolytic toxin,which is associated with severe skin infection.In China,the SCCmecIII-ST239 strains are disseminated in different geographic regions,accounting for > 75% of all MRSA isolates in some national studies.The epidemiology of MRSA has ever been changing,which results different drug resistant pattern in different continents,countries and regions.To explore the unique epidemiology and clinical infection characteristics of a region is a meaningful scientific practice.Based on the above research background,here we characterized 150 non-duplicate MRSA isolates collected from February 2012 to May 2013 in a tertiary hospital in Suzhou,Eastern China,to explore the molecular epidemiology.At the meantime,we collected the clinical information of patients infected by the MRSA strains and analyzed the clinical infection characteristics.[Objectives](1)To explore the drug resistance rate of MRSA in our hospital.(2)To explore the molecular epidemiology of MRSA in our hospital and explain the drug resistance mechanisms on molecular level.(3)To explore the clinical infection characteristics of MRSA infection in our hospital and provide experimental evidence for prevetion and treatment of MRSA infection.[Methods](1)Antibiotic susceptibility testing was performed using automated microdilution method.All the isolates were tested for mecA gene.(2)All isolates were characterized by spa typing,SCCmec typing,Representative genotypes were also subjected to multilocus sequence typing(MLST).When necessary,PFGE and MLVA typing will be used.(3)All isolates were detection of genes encoding Panton-Valentine leukocidin(PVL)and toxic shock syndrome toxin(TSST-1).(4)Clinical information of patients infected by the MRSA strains were collected.Clinical infection characteristics of MRSA infection were investigated by multivariate logistic regression analyses[Results](1)All the 150 MRSA isolates were mecA positive.All isolates were susceptible to vancomycin and linezolid.The percentage of resistance to both penicillin and oxacillin were 100.0%(150/150),while resistant tocefoxitin,piperacillin-tazobactam,ampicillin-sulbactam,ciprofloxacin,nitrofuantoin,erythromycin,clindamycin,trimethoprim-sulfame thoxazole and rifampin were 94.0%(141/150),98.0%(147/150),66.0%(99/150),64.0%(96/150),60.0%(90/150),74.0%(111/150),50.0%(75/150),72.0%(108/150),and 12.7%(19/150),respectively.(2)Molecular typing identified 11 clonal complex(CC)and 28 spa types,with the CC5-spa t002(29.3%)and CC239-spa t037(14.7%)being the most prevalent.SCCmec types II,III,IV and V were identified in 33.3%,21.3%,23.3% and 21.3% of all isolates,respectively.PVL genes(lukF/S-PV)were detected in 11.3% of all isolates and from 6 CCs(5,8,59,88,239,and 398).The TSST-1 gene(tst)was detected in 18.0% of the all isolates,predominantly in CC5(96.3%).All the tst-1-positve CC5 isolates were spa t002.(3)A total of 150 unique MRSA isolates from different specimens were included in this study.Among these,71 were collected from internal medicine wards(47.3%),48 from surgical wards(32.0%),and 31 from intensive care unit(ICU)wards(20.7%).All isolates were obtained from inpatients.60% of patients were more than 60 years old,with an average age of 63,and 74.0%(n=111)are male.A total of 87.3%(n=131)of the cases were hospital-onset.The majority of the isolates were from sputum(n=90,60.0%),followed by drainage(n=17,11.3%)and wounds(n=12,8.0%);4 isolates were from blood,and 5 were from urine,while the rest of the 22 isolates were from other sites.(4)In this study,18 patients died within 30 days of hospitalization,and the in-hospital 30-day mortality was 12.0%.We then compared the outcomes,clinical characteristics,and MRSA isolate genotypes within three major CCs(CC5,CC239 and CC59).In comparison to cases infected with CC239 or CC59 strains,patients with CC5 infections had significantly higher 30-day mortality(P<0.05).In addition,CC5 strains had a higher frequency of SCCmec type II and tst-1(P<0.01),and were more likely to be isolated from patients admitted to an ICU(P<0.05).We then compared the clinical and molecular characteristics between patients with different clinical outcomes(survival vs.death).The following factors were associated with 30-day mortality in univariate analysis: age,cancer,erythromycin resistance,belonging to CC5,harboring SCCmec II,spa t002,and presence of tst-1(P<0.05).In addition,ICU admission displayed border line significance(P=0.06).Notably,we observed 30-day mortality in patients infected with tst-1-positive MRSA was significantly higher than that inpatient infected with tst-1-negative MRSA(51.9% vs 3.3%,P<0.001).By contrast,no significant difference in 30-day mortality was observed between PVL-positive MRSA infected group vs.PVL-negative MRSA infected group(0.0% vs.13.5%,P=0.13).In multivariable analysis,factors independently associated with 30-day mortality included age greater than 60 years old(odds ratio [OR]=7.2,95% confidence interval [95% CI]=1.26-41.6,P=0.026),cancer diagnosis(OR=9.6,95% CI=1.4-65.7,P=0.02),and MRSA isolate carriage of tst-1(OR=62.5,95% CI=12.0-325.2,P<0.001).[Conclusions](1)Though the overall drug resistance of MRSA in this region was on high level,none of the tested strains was resistant to vancomycin and linezolid.(2)Our study revealed unique MRSA dissemination patterns in this region in comparison to those of other regions in China.Firstly,CC5 was the most prevalent clone complex in this region,the main genotype was SCCmec II-ST002,not SCCmec III-ST239.Secondly,spa t037 was more prevalent than spa t030 in our region,this result was different from previous investigation taken in other regions of China,which announced that spa t037 has replaced by spa t030.Our study revealed that MRSA dissemination patterns may be different in different hospitals and regions in China.(3)CC5 was the most prevalent clone complex in this region,especially,among them,the propotion of tst-1-positive-CC5-MRSA-II,spa t002 strains is high.The finding that tst-1-positive CC5 strains were associated with higher mortality highlights the need for strict infection control measures in order to prevent further spread of these strains in this region,as well as others.