Comparison of vanA gene mRNA levels between vancomycin-resistant Enterococci presenting the VanA or VanB phenotype with identical Tn1546-like elements.

Abstract

During the routine screening of vancomycin-resistant Enterococci faecium (VREfm) we found that VanA phenotype-vanA genotype and VanB phenotype-vanA genotype isolates had an identical Tn1546-like element structure. This study aimed to evaluate the genetic background and vanA gene expression to identify the mechanisms of the development of the VanB phenotype-vanA genotype VREfm. Twelve VREfm isolates were collected from a 1500-bed tertiary-care teaching hospital in Beijing. Genetic variations of the Tn1546-like element were determined by an overlapping polymerase chain reaction assay and sequencing. The genetic background wasdetermined by pulsed-field gel electrophoresis and mutilocus sequence typing. vanA gene expression was evaluated using a TaqMan quantitative real-time polymerase chain reaction. For the 12 isolates, six isolates with the VanA phenotype-vanA genotype and six with the VanB phenotype-vanA genotype were identified. According to the structure analysis of the Tn1546-like elements, our isolates were divided into two types. In the four isolates of type A, IS1542 and IS1216V were inserted into the orf2-vanR and vanX-vanY regions, respectively. In the eight isolates of type D, a similar insertion as type A occurred, except for an ISEfa4 insertion into IS1542. A significant difference in vanA gene expression was observed between the VanA and VanB phenotype isolates in type A, but not in type D. Mutilocus sequence typing and pulsed-field gel electrophoresis analysis showed that these isolates have a different genetic background. Our results indicated that the occurrence of the VanB phenotype-vanA genotype might not completely depend on the structure of Tn1546-like elements and vanA gene expression.