Morphological alterations of Staphylococcus aureus and Streptococcus pyogenes exposed to cefdinir, a new oral broad-spectrum cephalosporin.

Affinity for penicillin-binding proteins (PBPs) and the morphological alteration of Staphylococcus aureus 209-P JC and Streptococcus pyogenes C-203 exposed to cefdinir were studied. Although cefdinir was bactericidal against both strains, the extent of the decrease in colony-forming units (CFU) was similar in a fairly wide range of concentrations. Transmission electron microscopy of S. aureus 209-P JC revealed that cefdinir induced thickening of the cross wall and frequent cell lysis at low concentrations. The lytic sites were seen at the site of septum formation. In S. pyogenes C-203, cefdinir induced thickening of the peripheral wall and cross wall, and protoplast-like cells were observed during the incubation period. Cefdinir showed high affinity for all PBPs of S. aureus 209-P JC and S. pyogenes C-203. The drastic changes in the morphology of S. aureus and S. pyogenes were caused by binding of cefdinir to all PBPs.