Synergism between β-lactams and glycopeptides against VanA-type methicillin-resistant Staphylococcus aureus and heterologous expression of the vanA operon

Vancomycin resistance of Staphylococcus aureus NY-VRSA and VRSA-5 is due to acquisition of a vanA operon located in a Tn1546-like element. The vanA gene cluster of NY-VRSA contained one copy of insertion sequences IS1251 and IS1216V relative to that of VRSA-5. As evidenced by the nature of the late peptidoglycan precursors and by quantification of D,D-peptidase activities, the vancomycin resistance genes were efficiently expressed in both strains. Study of the stability and inducibility of glycopeptide resistance suggested that low-level glycopeptide resistance of NY-VRSA was most probably due to plasmid instability combined with a long delay for resistance induction. The activity of combinations of vancomycin or teicoplanin with oxacillin against the four VanA-type S. aureus strains already reported was tested by single and double disk diffusion, E-test on agar alone or supplemented with antibiotics, the checkerboard technique, and by determining time-kill curves. A strong synergism against the four clinical isolates, with fractional inhibitory concentration indexes from 0.008 to 0.024, was reproducibly observed between the two antibiotics by all methods. These observations indicate that cell wall inhibitors of the beta-lactam and glycopeptide classes exert strong and mutual antagonistic effects on resistance to each other against VanA-type methicillin-resistant S. aureus.