Objective: To explore the molecular types of methicillin-resistant Staphylococcus aureus (MRSA) strains present in major hospitals in Qingdao area, using pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) methods, trying to find out the epidemiological characteristics of these MRSA isolates. Correlation of the PFGE types with microbiological phenotypes and clinical data was also studied.
Methods: 360 isolates of MRSA were procured during 2003 to 2007 from major hospitals in Qingdao. PFGE technology was applied to comparatively analyze the chromosomal DNA digested with endonuclease SmaI. Comparison of DNA fragments patterns from each MRSA strain and cluster analysis were performed with the Bionumerics version '2.0' software. A dendrogram was generated using PFGE macrorestriction fragments on gel images. Data was used to predict the possibility of each PFGE type via SPSS software version 11.0, using the variables as predictors including groups on patient's age, gender, source and the site where MRSA was isolated. Antibiotic sensitivity patterns of these MRSA isolates were determined by K-B tests, and a correlation between these patterns and PFGE types was investigated. Housekeeping genes were amplified with PCR and sequenced in representative strains of variant PFGE types to identify their allelic profile.
Results: 5 types of PFGE patterns (M0-M4) were identified with M1 being the predominant and M2 next to it which was significantly correlated to the isolates from wounds. M3 type strains were mainly isolated from ICU wards and there were a few cases complied with M4 type with no correlated variant factors found in this study. A unique pattern of MRSA isolates with its M0 distinct from other types had not been reported. No significant association was found between PFGE individual types, gender or age groups. M1 and M2 types were the major proportional PFGE patterns among different hospitals. No vancomycin-resistant isolates were detected among 360 MRSA strains. No significant association was found between individual antibiotic resistance and specific PFGE types. Data from MLST analysis showed that the allelic profiles of M1 and M3 type strain had the same ST239 linage which was commonly present in China. For M2 and M4 representative strains, the allelic profiles were ST5 and ST240, respectively. ST45 and ST398 were corresponding to two PFGE patterns clustered as M0 type.
Conclusion: Nosocomial infection due to MRSA was evenly distributed among different age groups and no gender bias was observed. The PFGE types of MRSA strains isolated in major hospitals in Qingdao were highly correlated with the sources of isolates and ST239 isolate seemed the prevalent and widespread one. Strategies should be designed to further monitor and prevent or minimize the spread of ST5 MRSA isolates and the like, in Qingdao area.