In regard to glycopeptide resistance, there are nine resistance clusters (gene operons vanA, vanB, vanC, vanD, vanE, vanG, vanL, vanM, and vanN), three of which occur more commonly than others. The resulting Van phenotypes are the expression of the van gene operons in different enterococci strains. The VanA phenotype has an inducible high-level resistance to vancomycin as well as teicoplanin (encoded by the vanA gene). The VanB phenotype (encoded by two vanB genes) has moderate to high resistance to vancomycin only. The VanC phenotype (encoded by two vanC genes) demonstrates a non-inducible low-level resistance to vancomycin.
VanA and VanB are the most clinically significant phenotypes and are usually seen among E. faecalis and E. faecium isolates. VanC is both intrinsic and characteristic in E. gallinarum and E. casseliflavus. Because they are intrinsic rather than acquired, they represent a different impact on hospital epidemiology; definitive speciation can have significance for infection control purposes.
At the present time, both ampicillin and vancomycin resistance occur more frequently with E. faecium isolates than with E. faecalis. Most vancomycin-resistant E. faecium strains possess the vanA gene.
