| Objective The study was conducted to determine the drug susceptibility and analyze resistance mechanisms, and to detect antimicrobial-resistant genes and virulence genes among Staphylococcus aureus collected from bloodstream infections. To investigate the molecular epidemiological characteristics of Methicillin-resistant Staphylococcus aureus (MRSA), in order to control the spread of isolates in hospital.Methods A total of60non-repetitive Staphylococcus aureus causing bloodstream infections were isolated from January2006to August2011in the General Hospital of Tianjin Medical University. The bacteria identification and antimicrobial susceptibility tests were performed by VITEK-2compact automatic system. MRSA was identified using the disk diffusion method with cefoxitin. PCR was used to detect antimicrobial-resistant genes of mecA, ermA, ermB, ermC, aac(6’)/aph(2"), aph(3’)-Ⅲ, ant(4’4"), qacA and virulence genes of pvl, sea-see, tst. The PCR products were purified and following with sequencing. The DNA sequences obtained were compared with those in the National Center Biotechnology Information database. Molecular analysis of the MRSA strains was done using pulsed-field gel electrophoresis (PFGE) and staphylococcal cassette chromosome mec (SCCmec) typing to assess genetic relatedness.Results (1) All the60isolates were susceptible to vancomycin, linezolid, tigecycline, teicoplanin. The overall susceptible rates to moxifloxacin, rifampin, levofloxacin, cefoxitin, oxacillin were higher than78.0%, while to penicillin, erythromycin, clindamycin were lower than40.0%.13(21.7%) isolates were resistant to cefoxitin, which were confirmed to be MRSA.(2) The PCR indicated that13strains possessed mecA gene (21.7%), ermA, ermB, ermC, aac(6’)/aph(2"), aph(3’)-III, qacA were detected in10(16.7%),18(30.0%),18(30.0%),26(43.3%),6(10.0%) and3(5.0%) respectively. ErmA and qacA were found only in MRSA and ermB was showed only in Methicillin-susceptible Staphylococcus aureus (MSSA), and the positive rate of ermC, aac(6’)/aph(2"), aph(3’)-Ⅲ were higher in MRSA than MSSA. No ant(4’4") was found in all isolates.(3) Among the60isolates, pvl was positive in4(6.7%) and sea, seb, sec1, sed were detected in20(33.3%),3(5.0%),9(15.0%),7(11.7%) respectively. See and tst was negative in all strains. MRSA isolates only harbored sea and the positive rate of sea was higher than that in MSSA. Pvl, seb, sec1, and sed were non-detected in MRSA isolates but were present in MSSA isolates. The coexistence of sec1and sed was observed in6MSSA isolates, sea and sec; in2MSSA isolates and only1MSSA isolates harbored sea and sed. The result of sequencing confirmed that the PCR products were target genes.(4) SCCmec typing results showed that11MRSA isolates were SCCmecIII and typing in2MRSA isolates were indeterminable.(5) PFGE analysis of the13MRSA isolates produced four distinct genotypes. The PFGE pattern types were as follows:type A (2isolates), type A1(2isolates), type A2(1isolate), type A3(1isolate), type A4(2isolates), type A5(1isolate), type B (2isolates), type C (1isolate), and type D (1isolate).Conclusion The susceptible rates of Staphylococcus aureus collected from bloodstream infections to vancomycin, linezolid, tigecycline, teicoplanin were100.0%, and moxifloxacin, rifampin, levofloxacin, cefoxitin, oxacillin exhibited high susceptibility. Nevertheless penicillin, erythromycin, clindamycin were in low susceptibility. Staphylococcus aureus harbored various kinds of antimicrobial-resistant genes and virulence genes. Antimicrobial-resistant genes carried by MRSA were more than MSSA, while virulence genes were less than MSSA. The presence of antimicrobial-resistant genes of erm and aac(6’)/aph(2") were primarily correlated with Staphylococcus aureus resistant to macrolides and aminoglycosides respectively. The PFGE results demonstrated the small-scale clonal spread of MRSA, mainly involving SCCmec type III. |
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