Purpose: Community-associated Staphylococcus aureus  (CA S. aureus ) is the most common causative pathogen of the skin and soft tissue infections (SSTIs). This study aims to determine clonal distribution, virulence factors of CA S. aureus clinical isolates from purulent SSTIs in Beijing, China.
Materials and methods: CA-S. aureus  isolates were collected from 115 outpatients with purulent SSTIs from the department of dermatology from April 2015 to April 2017. Multilocus sequence typing and Staphylococcus cassette chromosome mec typing were performed to explore molecular characteristics. Phylogenetic analysis of 16S rRNA of dominant S. aureus  isolates was performed using MEGA-X software. Virulence genes were detected by PCR, while biofilm formation was evaluated by a microtiter plate method. The antimicrobial susceptibility was tested by an automatic VITEK system.
Results: Forty-four CA-S. aureus  isolates identified from SSTIs contain 9 methicillin-resistant S. aureus  (MRSA) isolates (20.4%) and 35 methicillin-susceptible S. aureus  isolates (MSSA) (79.6%). The dominant sequence types (STs) were ST22 (40.9%) and clonal complex 59 (CC59; 77.8%) in Community-associated methicillin resistant methicillin-resistant S. aureus . 27.8% of ST22 isolates were homologous to the epidemic ST22 EMRSA-15 in Europe. The prevalence of virulence genes lukS/lukF, tst-1, etA, edinA, icaA, and icaD  was 50%, 93.2%, 4.5%, 4.5%, 100%, and 100%, respectively. All CC59 isolates exhibited stronger biofilm-forming capability than ST22 clones. Among the MSSA subgroup, the poor biofilm producers had significantly higher sensitivity to sulfamethoxazole/Trimethoprim.
Conclusion: The dominant epidemic clone PVL⁺ ST22 MSSA containing tst-1  occurs in Beijing, indicating that a PVL⁺ ST398 clone which was previously predominant in this district had been replaced by a new clone.
Keywords: community-associated Staphylococcus aureus , skin and soft tissue infection, biofilm formation, MLST-genotyping