Molecular Epidemiologic Analysis of vanA-Containing Vancomycin-resistant Enterococci

Abstract

Background : Vancomycin-resistant enterococci (VRE) have been increasingly isolated worldwide as a nosocomial pathogen. To target infection control, epidemiologic investigations of VRE should include analysis of the resistance gene in addition to typing of strains. We performed molecular characterization of the vanA resistance gene to evaluate the inter or intraconstitutional spread. Methods : Twenty isolates of VanA VRE from the Centers for Disease Control and Prevention(CDC) and 17 from Ajou University Hospital(AUH) were investigated. Minimum inhibitory concentrations of vancomycin, teicoplanin, and ampicillin were tested by the agar dilution method. Pulsedfield gel electrophoresis(PFGE) and long PCR-restriction fragment length polymorphism (long PCR-RFLP) were performed. The long PCR negative strains were typed by ORF1-, vanS-vanH-, vanX-, vanY-vanZ-, vanZ-, and IS1216-specific PCRs. Filter matings were performed by using rifampin-resistant, fusidic acid-resistant E. faecalis JH2-2 as the recipient Results : The PFGE from the VRE of the CDC showed 15 patterns including 4 clusters and PFGE from isolates of AUH revealed 6 patterns including 3 clusters. Tn 1546 amplicons were detected in 18 of 20 (90%) CDC strains and 16 of 17 (94%) AUH strains. RFLP of Tn 1546 amplicons revealed 5 different patterns in the VRE of the CDC strains, and 2 patterns in the VRE of the AUH strains. The mean transfer efficiency of the CDC and the AUH strains are 3.0×10-8 and 4.9×10-5 transconjugant/donor, respectively. Conclusions : Molecular typing of isolates from the CDC suggests the horizontal spread of vanA genes among genetically diverse strains. Analysis of the VRE from the AUH shows a mixed pattern with clonal dissemination of strains and horizontal transfer of vanA.