Binding of beta-lactam antibiotics to penicillin-binding proteins of Staphylococcus aureus and Streptococcus faecalis: relation to antibacterial activity.

The binding of 14 structurally diverse beta-lactam antibiotics to penicillin-binding proteins of Staphylococcus aureus and Streptococcus faecalis was studied, and the results were examined in the context of the antibacterial activity of the compounds. Penicillin-binding proteins 1 (molecular weight, 87,000) and 3 (molecular weight, 75,000) of S. aureus and penicillin-binding proteins 1 (molecular weight, 105,000) and 3 (molecular weight, 79,000) of S. faecalis bound beta-lactam antibiotics at concentrations comparable to minimum inhibitory concentrations and might therefore be essential. The low affinity of S. faecalis penicillin-binding proteins, relative to that of S. aureus penicillin-binding proteins, toward most beta-lactam antibiotics is probably responsible for the resistance of the former organism to most of these compounds.