To reveal the association between the vanM gene and different glycopeptide resistance phenotypes in Enterococcus faecium, drug susceptibility testing and sequencing analysis were conducted on four E. faecium strains carrying the vanM gene.

The broth microdilution method was employed to determine the susceptibility of four E. faecium strains to glycopeptide antibiotics. Polymerase chain reaction (PCR) was performed to detect the vancomycin-resistant genotypes, while population analysis profiling (PAP) was used to identify the presence of heteroresistant strains among vancomycin-susceptible isolates. Quantitative real-time PCR (qRT-PCR) was conducted to analyze the relative expression level of the vanM gene in E. faecium, and genomic sequencing was carried out for alignment of the resistance gene cluster.

All four E. faecium strains carried the vanM gene. Strain SZD1 was susceptible to vancomycin, whereas SZD2 was identified as a vancomycin-heteroresistant strain. Strain SZD3 exhibited a high relative expression level of the vanM gene and showed resistance to vancomycin. In addition, strain SZD4 harbored both vanA and vanM genotypes, conferring high-level resistance to vancomycin.

E. faecium strains harboring the vanM gene can exhibit diverse glycopeptide resistance phenotypes, and such variations may be associated with the structure, copy number, and expression level of the vanM gene cluster, as well as the coexistence of other resistance genes.