Heteroresistance in vancomycin-variable Enterococcus faecium ST80 isolate

Purpose: To investigate the gradual genetic changes occurred in heteroresistant vancomycin-variable (VVE) Enterococcus faecium ST80 isolate - belonging to the vanA ST80 nosocomial clone although recovered from marine environment - contributing to its reversion to a resistant phenotype after exposure to increasing concentrations of vancomycin. Methods: WGS of the parental strain and three revertants obtained by exposure to vancomycin was performed. vanA copy number and difference in vanA expression between VVE and revertants were evaluated by RT-qPCR. Heteroresistance was demonstrated by vancomycin E-test and population analysis profiling. Results: The E. faecium JSEG15 isolate, susceptible to vancomycin although carrying a Tn1546-like transposon on 36-kb plasmid, gave rise to three different resistant mutants following gradual passages on increasing vancomycin concentrations. Revertants showed increased vancomycin MICs, with the highest resistance in JSEG15-rev3 (MIC = 128 mg/L). All revertants presented a deleted Tn1546-like on a 34-kb plasmid and JSEG15-rev2 and JSEG15-rev3 also showed an additional chromosomal copy of the vanRS and vanHAX clusters, and higher vanA gene copies and expression. A mutation in the chromosomal D-Ala-D-Ala ligase gene was identified in JSEG15-rev3 which also transferred the vanA carrying plasmid by in vitro conjugation. The parental strain and all revertants exhibited heteroresistance, with higher frequency of resistant subpopulations in the revertants. Conclusion: These findings emphasize the dynamic genetical changes enabling VVE to regain full resistance, underscoring the need for vigilant treatment strategies to address enterococcal infections and for monitoring the spread of clinically relevant strains in the environment for public health protection.