Inducible vancomycin resistance is common in porcine Enterococcus gallinarum and E. casseliflavus isolates

Abstract

Enterococcus gallinarum and E. casseliflavus possess intrinsic vancomycin resistance, which is mediated by vanC-1 and vanC-2 gene clusters, respectively. The VanC resistance phenotype is expressed inducibly or constitutively. This study characterized the expression types for vancomycin resistance in 23 porcine E. gallinarum and 19 porcine E. casseliflavus isolates with vancomycin minimum inhibitory concentrations between 4 and 16 μg/ml. The vanSC and vanSC-2 genes were sequenced and the deduced amino acid sequences were compared to those of the inducible and constitutive resistance reference strains. Analysis of growth curves revealed that the resistance phenotype in all isolates was induced by vancomycin. In the presence of vancomycin, lagging periods of these bacterial strains were 3 hours longer than when they were cultured in media without the drug. Transcriptional analysis using reverse transcription-polymerase chain reaction revealed that vanC-1 and vanC-2 genes were expressed by E. gallinarum and E. casseliflavus, respectively, when they were grown in the presence of vancomycin. Thus, the vanC-1 and vanC-2 mRNA expression were inducible. Alignment of the VanSC and VanSC-2 protein sequences of the Van inducible and constitutive strains revealed conserved glycine residues at positions 320 and 296 in the inducible strains of E. gallinarum and E. casseliflavus, respectively. These results suggest that VanC enterococci from Thai swine may commonly express inducible vancomycin resistance phenotype and the inducible vancomycin resistance is associated with the conserved glycine residues of VanSc proteins.