The in vitro antibacterial activity of ceftriaxone against Streptococcus pyogenes is unrelated to penicillin‐binding protein 4

The in vitro activities of penicillin and ceftriaxone were compared against 29 strains of Streptococcus pyogenes with the result that ceftriaxone showed greater activity than penicillin. The morphological changes induced by 1/2 and 1× MIC concentrations of penicillin and ceftriaxone, respectively, were very similar using scanning electron microscopy. Competitive binding studies using ‘cold’ penicillin or ceftriaxone as inhibitors ofradiolabeled penicillin binding demonstrated that ceftriaxone had a very low affinnity for penicillin binding protein (PBP) 4 compared to that of penicillin. Since ceftriaxone had greater antibacterial activity, this suggests that PBP 4 may not be important to the in vitro of ceftriaxone. In contrast, the IC50 for ceftriaxone was much lower (> 200 fold) for PBPs 2 and 3 compared to PBP 4, suggesting greater avidity of these high molecular mass PBPs for ceftriaxone. These data may at least in part explain the superior in vitro activity of ceftriaxone compared to penicillin against S. pyogenes. These data, together with the observation that PBP 1 was saturated at a lower concentration of penicillin than any of the other PBPs, suggest that the inhibition of PBPs 1, 2, and 3 mediates the bactericidal activity of beta‐lactam antibiotics against group A streptococci.