VanA-Type Staphylococcus aureus Strain VRSA-7 Is Partially Dependent on Vancomycin for Growth

VanA-type Staphylococcus aureus strain VRSA-7 was partially dependent on glycopeptides for growth. The vanA gene cluster, together with the erm(A) and the ant(9)-Ia resistance genes, was carried by the ca. 35- to 40-kb conjugative plasmid pIP848 present at five copies per cell. The chromosomal ddl gene had a mutation that led to a N308K substitution in the D-Ala: D-Ala ligase that resulted in a 1,000-fold decrease in activity relative to that of strain VRSA-6. Strain VRSA-7 grown in the absence or in the presence of vancomycin mainly synthesized precursors ending in D-Ala-D-Lac, indicating that the strain relied on the vancomycin resistance pathway for peptidoglycan synthesis. Greatly enhanced growth in the presence of glycopeptides and the absence of mutations in the genes for VanR and VanS indicated the inducible expression of resistance. Thus, a combination of loose regulation of the vanA operon by the two-component system and a gene dosage effect accounts for the partial glycopeptide dependence of VRSA-7. Since peptidoglycan precursors ending in D-Ala-D-Lac are not processed by PBP 2', the strain was fully susceptible to oxacillin, despite the production of a wild-type PBP 2'.