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

Background/Purpose 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 Tn 1546 -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. Methods Twelve VREfm isolates were collected from a 1500-bed tertiary-care teaching hospital in Beijing. Genetic variations of the Tn 1546 -like element were determined by an overlapping polymerase chain reaction assay and sequencing. The genetic background was determined by pulsed-field gel electrophoresis and mutilocus sequence typing. vanA gene expression was evaluated using a TaqMan quantitative real-time polymerase chain reaction. Results 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 Tn 1546 -like elements, our isolates were divided into two types. In the four isolates of type A, IS 1542 and IS 1216V 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 ISE fa4 insertion into IS 1542 . 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. Conclusion Our results indicated that the occurrence of the VanB phenotype– vanA genotype might not completely depend on the structure of Tn 1546 -like elements and vanA gene expression.