Beta-lactam antibiotics are able to inhibit the growth of Gram-positive bacteria by interfering with penicillin-binding proteins (PBPs), suppressing transpeptidase and transglycosylase reactions. This prevents peptidoglycan synthesis, inhibiting the bacteria from growing.
As peptidoglycan is unique to bacterial cells, the beta-lactam class of antibiotics are highly selective, with a broad spectrum of activity against bacteria.
However, resistance to these antibiotics is now widespread due to overuse (both clinically and in agriculture). Beta-lactamase enzymes are able to break apart the beta-lactam ring, preventing the antibiotic from inhibiting peptidoglycan synthesis successfully.