Abstract
Background: Vancomycin is a first-line therapy for C. difficile infection (CDI) and the emergence of resistance would have significant clinical consequences. As part of ongoing C. difficile surveillance activities, we identified a vancomycin resistance operon in a PCR ribotype 033 strain of C. difficile (AI0499), isolated from an Australian veal calf at slaughter.
Objectives: We sought to investigate the genomic context for this operon and determine any phenotypic effect on vancomycin activity in vitro.
Methods: The genome of strain AI0499 was sequenced and investigated using short-read sequence typing, de novo assembly and comparative genomic analysis. In vitro susceptibility to vancomycin was determined using agar and Etest methodologies.
Results: AI0499 harboured a cluster of seven vancomycin resistance genes (vanXB, vanB, vanHB, vanW, vanYB, vanSB and vanRB) showing >99% sequence identity to the vanB2 operon frequently found in vancomycin resistant enterococci (VRE). The operon was located on a ~42 Kb element showing significant homology and synteny to Tn1549, a conjugative transposon linked with the emergence and global dissemination of VRE. Notably, strain AI0499 did not show any reduced susceptibility to vancomycin (MIC 1 mg/L), possibly as a result of an aberrant vanRB gene encoding a cytoplasmic response regulator.
Conclusions: Here we present the first description of a vanB-like element in C. difficile, carried on a transposable element of clinical significance. Although, phenotypically silent in AI0499, its presence is alarming and if expressed in vivo could have dramatic consequences. These data further confirm gut microbiota are a reservoir of important vanB-like resistance operons.