Rapid depletion of free vancomycin in medium in the presence of beta-lactam antibiotics and growth restoration in Staphylococcus aureus strains with beta-lactam-induced vancomycin resistance.

A class of methicillin-resistant Staphylococcus aureus strains shows vancomycin resistance in the presence of beta-lactam antibiotics (beta-lactam-induced VAN-resistant methicillin-resistant S. aureus [BIVR]). Two possible explanations may be offered: (i) vancomycin in culture medium is depleted, and (ii) the D-Ala-D-Ala terminal of the peptidoglycan network is replaced with D-Ala-D-lactate. We tested these hypotheses by quantifying free vancomycin in the medium through the course of cell growth and by PCR amplification of the van genes. Growth of the BIVR cells to an absorption level of approximately 0.3 at 578 nm required about 24 h in the presence of vancomycin alone at the MIC (4.0 microg/ml). However, growth was achieved in only about 10 h when 1/1,000 to 1/2,000 the MIC of beta-lactam antibiotic was added 2 h prior to the addition of vancomycin, suggesting that the beta-lactams shortened the time to recovery from vancomycin-mediated growth inhibition. Free vancomycin in the culture medium decreased to 2.3 microg/ml in the first 8 h in the culture containing vancomycin alone, yet cell growth was undetectable. When the vancomycin concentration dropped below approximately 1.5 microg/ml at 24 h, the cells began to grow. In the culture supplemented with the beta-lactam 2 h prior to the addition of vancomycin, the drug concentration continuously dropped from 4 to 0.5 microg/ml in the first 8 h, and the cells began to grow at a vancomycin concentration of approximately 1.7 microg/ml or at 4 h of incubation. The gene encoding the enzyme involved in D-Ala-D-lactate synthesis was undetectable. Based on these results, we concluded that BIVR is attributable mainly to a rapid depletion of vancomycin in the medium triggered or promoted by beta-lactam antibiotics.