| Objectives:To investigate the basic law of MRSA evolution and describe evolutionary pathways of different MRSA clones. To determine the risk factors and infection mortality associated with hVISA/VISA infections. To study the resistance genes associated with hVISA.Methods:A total of 466 nonduplicate S. aureus isolates including 302 MRSA and 164 methicillin susceptible (MSSA) isolates recovered from 1994 to 2008 were characterized by staphylococcal cassette chromosome mec (SCCmec) typing, spa typing, pulsed-field gel electrophoresis (PFGE), and multilocus sequence typing (MLST). hVISA isolates were screened by two agar screening methods BHIT5 (brain heart infusion containing teicoplanin 5μg/ml) or BHIV6 (brain heart infusion containing vancomycin 6μg/ml) and macroEtest method (MET), and were confirmed by modified population analysis profile-area under the curve (PAP-AUC). A retrospective case control study of patients with hVISA infection or VSSA infection was carried out and statistical analysis was performed using student's t test, Mann-Whitney test,χ2test and fisher exact test. The agr transcription level of hVISA was determined by real time PCR.Results:The 302 MRSA isolates were classified into 12 spa types and 9 STs. During the years of 1994-2000, the most predominant MRSA clone was ST239-MRSA-Ⅲ-spa t037. Since 2000, ST239-MRSA-Ⅲ-spa t030 has rapidly replaced t037, and become the major clone. Another clone, ST5-MRSA-Ⅱ-spa t002 emerged in 2002 and constantly existed at a low prevalence. The 164 MSSA isolates were classified into 62 spa types and 40 STs. ST398 was the most common MLST type in MSSA, followed by ST59, ST7, ST15 and ST1. Several MSSA genotypes including ST398, ST1, ST121 and ST59 were identical with well-known epidemic CA-MRSA isolates. MLST eBURST analysis revealed that ST5, ST59 and ST965 clones co-existed in both MRSA and MSSA, which suggested these MRSA clones might evolve from MSSA by the acquisition of SCCmec. Two pvl positive ST59-MRSA-Ⅳisolates were identified as CA-MRSA according to the clinical data. The percentage of isolates with a vancomycin MIC≥1.0μg/ml in hVISA group was significantly higher than the percentage of VSSA group (80% versus 24.9%, p<0.001). We could not demonstrate a significant difference in clinical outcomes and infection mortality between hVISA group and VSSA group. However, the admission days of hVISA group were longer than of VSSA group (P=0.025). Compared with the corresponding VSSA, the RNA III transcription levels in 9 of 10 hVISA significantly reduced, which decreased to 1,000 times at the largest.Conclusion:ST239-MRSA-Ⅲ-spa t037 clone was replaced by the emerging ST239-MRSA-Ⅲ-spa t030 clone, indicating a rapid change of MRSA at a tertiary care hospital of China over a fifteen-year period. Compared with VSSA, hVISA group has a higher average vancomycin MIC but still sensitive to many new antimicrobial agents such as daptomycin, linezolid, ceftobiprole and tigecycline. The infection of hVISA was not associated with vancomycin treatment failure, but the admission days of hVISA group were longer than of VSSA group. Prospective cohort study is necessary to reveal the correlation between hVISA and infection mortality. The down-regulation of agr was associated with reduced vancomycin susceptibility in Staphylococcus aureus. The detailed resistance mechanism of hVISA will be futher studied. |
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