Correlation between the binding of beta-lactam antibiotics to Staphylococcus aureus and their physical-chemical properties.

The rate of (14)C-benzylpenicillin (penicillin G) binding to Staphylococcus aureus Oxford cells increased with increasing hydrogen ion concentration. The extent of inhibition of (14)C-penicillin G binding caused by a competing (12)C-beta-lactam antibiotic is a function of hydrogen ion concentration and can be correlated both with net charge of a competing (12)C-molecule and net charge of the S. aureus cell at a given pH. The ability of a beta-lactam antibiotic to compete for (14)C-penicillin G-binding sites can generally be correlated with its hydrophobic nature. It is proposed that, although semisynthetic cephalosporins are chemically less reactive than penicillins, they are superior to benzylpenicillin in their ability to permeate the outer surface of the Staphylococcus cell wall and irreversibly bind to reactive sites.