Ultrastructural localization of penicillin-binding sites in Staphylococcus aureus using penicillin G-BSA-gold

Penicillin G (PcG) conjugated to bovine serum albumin (BSA) was found to retain antibacterial activity against Staphylococcus aureus as judged by growth inhibition and by ultrastructural analyses of the PcG-BSA-treated bacterium. Using the PcG-BSA conjugated to colloidal gold, binding sites of PcG were localized in S. aureus with electron microscopy using a postembedding labeling technique. To our surprise, labeling was observed both on the cell wall and in the cytoplasm of S. aureus. A part of the labeling observed may be attributed to penicillin-binding proteins (PBPs) or their precursors. Also there may be other molecules which bind with PcG in the cell wall and cytoplasm of S. aureus.