Poster Presentation Australian Society for Microbiology Annual Scientific Meeting 2024

Molecular and phenotypic characterisation of vancomycin variable Enterococcus faecium causing bacteraemia in Australia (#18)

Auriane N Form 1 , Geoffrey W Coombs 1 2 3 , Princy Shoby 1 , Denise A Daley 2 3 , Shakeel Mowlaboccus 1 2 3
  1. Antimicrobial Resistance and Infectious Diseases Research Laboratory, Murdoch University, Murdoch, WA, Australia
  2. Department of Microbiology, PathWest Laboratory Medicine WA, Fiona Stanely Hospital, Murdoch, WA, Australia
  3. Australian Group on Antimicrobial Resistance, Fiona Stanley Hospital, Murodch, WA, Australia

Introduction

In Australia, vancomycin-resistant E. faecium (VREfm) causing bacteraemia harbours the vanA and/or vanB operon(s). Unlike vanB-positive VREfm, vanA-positive VREfm are additionally teicoplanin-resistant. Vancomycin-variable E. faecium (VVEfm) are vanA- or vanB-positive E. faecium which are phenotypically susceptible to vancomycin due to mutations in the van operon. Some VVEfm can revert to VREfm when exposed to vancomycin in vivo, resulting in treatment failure.

Objective

The aim of this study is to determine the genetic diversity of VVEfm causing bacteraemia in Australia and to test the effect of vancomycin exposure on VVEfm in vitro.

Methods

Genomic DNA of 32 VVEfm isolates causing bacteraemia in Australia during 2016-2020 were sequenced on the Illumina® NextSeq™ 500 platform and analysed using various bioinformatics tools. Antimicrobial susceptibility testing was performed on the Vitek® 2. Vancomycin minimum inhibitory concentration (MIC) was determined by broth microdilution and interpreted using the EUCAST breakpoints. Exposure to increasing concentrations of vancomycin was performed in an overnight broth culture.

Results

A total 22 (68.75%) vanA-positive and 10 (31.25%) vanB-positive VVEfm were identified. The vanA-positive isolates belonged to four sequence types (STs), two of which harboured more than one isolate, ST1424 (n=15) and ST1421 (n=5). The vanB-positive isolates belonged to eight STs, of which only ST796 (n=3) harboured more than one isolate. All vanA operons harboured at least one insertion sequence (IS) (IS1542, IS1216E, IS1216, IS6, IS200/IS605, and ISEfa4) and truncated or absent genes. The vanB operons harboured either point mutations, IS elements (IS110 and ISL3), truncated or absent genes. Twelve isolates (36.4%), including 10 vanA-positive and 2 vanB-positive VVEfm, became vancomycin-resistant on exposure to vancomycin, with a minimum 256-fold increase in vancomycin MIC. Seven vanA-positive VVEfm mutants and one vanB-positive VVEfm mutant became teicoplanin resistant. All VVEfm mutants remained multi-drug resistant to the same antimicrobial classes. Some of the vancomycin-resistance reversion mechanisms were previously reported; however, novel mechanisms were also found.

Conclusions

vanA- and vanB-positive VVEfm causing bacteraemia in Australia are genetically diverse. Some VVEfm can revert to a vancomycin-resistant phenotype when exposed to vancomycin, and some may even acquire teicoplanin resistance.