Molecular Epidemiological Study Of Methicillin Resistant Staphylococcus Aureus

Objectives:To investigate the molecular epidemiological characteristic of the methicillin-resistant Staphylococcus aureus (MRSA) causing nosocomial infections in Tianjin Nankai Hospital, Pulsed field gel electrophoresis (PFGE) analysis and Staphylococcal Cassette Chromosome mec (SCCmec) genotyping were performed.Methods:(1) The relevant bacterial strains isolated from clinical samples were identified with biochemical fermentation reaction for staphylococci, plasma coagulase test for Staphylococcus aureus, and cefoxitin susceptibility test for MRSA. The Staphylococcus aureus ATCC25923 was used as reference strain.(2) According to NCCLS (2004 edition), K-B disk diffusion method was employed to examine the susceptibilities of the MRSA strains to 14 kinds of antibiotics.(3) The genetic features of the MRSA strains were characterized by PFGE analysis, mecA gene detection and SCCmec genotyping.Results:(1) Infection sites:Among the 40 MRSA strains,28 were isolated from sputum, which was 70.0% of the total isolates,7 (17.5%) from ascite or peritoneal drainage fluid,2 (5.0%) from each of blood and wound drainage fluid, and 1 (2.5%) from throat swab.(2) SCCmec genotyping:the 40 MRSA isolates were divided into 4 types with the multiplex PCR methods developed by Oliveira and Zhang. Among the 4 types, the 33 strains of typeâ…¢consisted of the dominant group, which accounted for 82.5%, followed by 5 (12.5%) for typeâ…¤,1 (2.5%) for each of typesâ… andâ…£.(3) PFGE analysis:the 40 strains were divided into five types, from A to E; 21 strains (52.5%) were for type A,8 (20.0%) for type B,6 (15.0%) for type D,4 (10.0%) for type C,1(2.5%) for type E.(4) Relationship between PFGE type and antimicrobial resistance spectrum:All types of MRSA were identically resistant to Clindamycin, Cefoxitin, Oxacillin Erythromycin and Penicillin, nevertheless the type-defined resistance pattern were showed by each of the type groups; for instance, the isolates of type A were resistant to 6, type D to 5, types C and E to 4, and type B to 2 other antimicrobials.(5) Based upon the time and location distribution of the MRSA strains in the hospital, the occurrence and spread of nosocomial infections with MRSA could be clearly identified and tracked depending on the genetic characteristics of the MRSA isolates.Conclusions:(1) The MRSA strains used in this study were obtained from different infection sites of the hospitalized patients, the sputum was the major source. This phenomenon was obviously related with the fact that respiratory tract was the main colonization site for staphylococci.(2) From the year 2007 to 2008, only 40 strains of MRSA were collected from the patients with nosocomial infection and the 40 strains belonged to 5 different PFGE types, indicating that the infections caused by MRSA in Tianjin Nankai hospital were sporadic.(3) All 40 MRSA strains were multi-resistant to antibiotics, but the strains showed the resistant pattern common to all PFGE types as well as the type-specific phenotypes. The difference in resistance spectrum was significant for the clinical application of antimicrobial agents.(4) In this study, SCCmec III was the major type among the studied isolates, which was consistent with the genetic feature shared by the nosocomial MRSA strains. By the combination of PFGE typing and SCCmec typing, MRSA strains could be characterized from different aspects at genetic level. Therefore the source, spread route and distribution of MRSA strains could be readily determined and tracked, from which the reliable evidence could be provided for surveillance and control of nosocomial infections.