Mutations leading to increased levels of resistance to glycopeptide antibiotics in VanB-type enterococci.

The vanB gene cluster mediates glycopeptide resistance by production of peptidoglycan precursors ending in the depsipeptide D-alanyl-D-lactate (D-Ala-D-Lac) instead of D-Ala-D-Ala found in susceptible enterococci. Synthesis of D-Ala-D-Lac and hydrolysis of D-Ala-D-Ala is controlled by the VanR(B)S(B) two-component regulatory system that activates transcription of the resistance genes in response to vancomycin but not to teicoplanin. Two substitutions (A3C-->G or D168-->Y) in the VanS(B) sensor kinase resulted in induction by teicoplanin, indicating that the N-terminal domain of the protein was involved in glycopeptide sensing. A substitution (T237-->K) located in the vicinity of the putative autophosphorylation site of VanS(B) (H233) was associated with a constitutive phenotype and affected a conserved residue known to be critical for the phosphatase activity of related kinases. A mutant producing an impaired host D-Ala:D-Ala ligase required vancomycin for growth, since D-Ala-D-Lac was only produced under inducing conditions. The ddl and vanS(B) mutations, alone or in combination, resulted in various resistance phenotypes that were determined by the amount of D-Ala-D-Ala and D-Ala-D-Lac incorporated into peptidoglycan precursors under different inducing conditions.