Characteristics And Transmission Mechanism Of Methicillin-resistant Staphylococcus Aureus Lineages In China

Methicillin-resistant Staphylococcus aureus(MRSA)is one of the principal pathogens in healthcare-associated infections,and its high rate of isolation is worrisome.The emergence of community-acquired MRSA(CA-MRSA)with high virulence potential posed challenges for infection control.There is still no systematic epidemiological study in China on the prevalence of CA-MRSA and its transmission mechanism.On the other hand,some important CA-MRSA clones had emerged in China,but researches were limited.CA-MRSA Sequence type(ST)88 was usually reported in Africa and Asia,research on the virulence and genome of this clone was relatively limited.ST398 is the major clone of Livestock associated MRSA.Surgical site infections(SSI)caused by MRSA ST398 were rarely reported.In the first part,Staphylococcus aureus isolates and clinical data from 22 tertiary hospitals were collected through a multicenter prospective study,and isolates were subjected to antimicrobial susceptibility testing,whole genome sequencing and data analysis.A total of 840 community-onset Staphylococcus aureus(CO-SA)isolates and 1,060 healthcare-onset Staphylococcus aureus(HO-SA)isolates were included in the epidemiological analysis.236 CO-MRSA isolates(including 105 CA-MRSA isolates)and 235 HO-MRSA isolates were included in genomic analysis.The overall MRSA prevalence rate was 40.16%.The prevalence rate of CO-MRSA and CA-MRSA among CO-SA was 28.1%and 12.55%respectively.Among HO-SA,HO-MRSA accounted for 49.72%.The prevalence rate of MRSA varied among different hospitals and regions.The susceptible rates of fluoroquinolones,gentamicin,erythromycin,clindamycin and tetracycline to MRSA were less than 50%.For clindamycin and erythromycin,the resistant rate of CO-MRSA was higher than HO-MRSA.Vancomycin,teicoplanin and linezolid were 100%sensitive to all isolates.The main infection type of CA-MRSA was skin and soft tissue infection(43.81%),while for HO-MRSA was respiratory infection(50.85%).In CA-MRSA infection,the proportion of infants was higher,and the average age was lower.The main epidemic clone of CO-MRSA was ST59,accounting for 36.86%,followed by ST239 and ST5.The distribution of clone type was different between provinces and cities,and there were also emerging clones like ST45 and ST398.The main epidemic clone of CA-MRSA was ST59,accounting for 43.81%,followed by ST630,ST239,ST338 and ST5.For HO-MRSA,ST239 accounted for 32.34%,followed by ST5 and ST59.ST5 was the main clone in East China,and the number of ST59 in North China,South China and Central China increased to be comparable to ST239.Core-genome multilocus sequence typing(cgMLST)indicated genetic polymorphism among CO-MRSA isolates.In HO-MRSA clone ST5 and ST239,there were clusters in the same hospitals,indicating clonal dissemination.In the phylogenetic tree,there is no significant demarcation between CA isolates and hospital-acquired community-onset(HA-CO)isolates,indicating that there was limitation for MRSA classification by using clinical data.The same clone complex isolates,rather than the same ST isolates,were more closely related,indicating that the analysis based on cgMLST had better resolution.In the second part,ST88 CA-MRSA strain SR434,isolated from an outpatient with skin and soft tissue infection,was subjected to whole genome sequencing,antimicrobial susceptibility testing,mouse skin infection model and hemolysis analysis to identify its virulence and resistance determinants.MRSA strain SR434 is resistant to clindamycin,erythromycin and fosfomycin.Four plasmids with resistance genes were identified in this strain,including a 20,658 bp blaZ-carrying plasmid,a 2473 bp ermC-carrying plasmid,a 2622 bp fosB7-carrying plasmid and a 4817 bp lnuA-carrying plasmid.This strain contains staphylococcal cassette chromosome mec type IV and does not contain arginine catabolic mobile element or Panton-Valentine-Leukocidin.SR434 harbors genomic islands vSaa,vSaβ,vSay and ΦSa3 and pathogenicity islands vSa2 that carries genes encoding toxic shock syndrome toxin 1,superantigen enterotoxin C and superantigen enterotoxin L.Mouse skin infection model results showed that SR434 had similar virulence potential causing invasive skin infection as a PVL-negative epidemic Korea clone HL1(ST72).CA-MRSA strain of ST88 lineage might be a great concern for its high virulence.In the third part,two MRSA strains of ST398 were identified from surgical site infections in China.Strains were subjected to antimicrobial susceptibility testing,whole-genome comparison,hemolysis analysis and mouse skin infection model.SSI MRSA strains were both isolated from orthopedic patients who were re-admitted to hospital.FY20 was resistant to cefoxitin,clindamycin and erythromycin and contains blaZ,mecA and ermC genes,while FY22 only contains blaZ and mecA genes.They carry same virulence genes as SSTI ST398 strains.Erythrocyte lysis of SSI strains was significantly stronger than other strains.Mouse skin infection model results showed that SSI strains had similar potential causing invasive skin infection as epidemic CA-MRSA strain HL1.Genetic analysis and clinical data from SSI strains suggested that they were human-related but sporadic.MRSA ST398 isolates has emerged in hospital settings and caused surgical site infections with poor treatment outcome.Careful monitoring of the epidemiology about MRSA ST398 and implementation of infection control policies are important.In conclusion,this study investigated the prevalence of MRSA through a national multicenter prospective study for the first time,analyzed the characteristics of antimicrobial resistance phenotype,infection type and molecular typing of CO-MRSA,CA-MRSA and HO-MRSA,and explored the transmission mechanism.Simultaneously,the whole-genome and virulence characteristics of MRSA ST88 strain and transmission mechanism of ST398 strains isolated from surgical site infection were described.