The pneumococcus and the mouse protection test: importance of the lag phase in vivo.

During development of a mouse protection test with a pneumococcus type 3 we noted the importance of the immediate post-challenge lag phase of the bacteria in vivo for the effective timing of treatment with different antibiotics. The lag phase, lasting 6-8 h after intraperitoneal challenge and parallel in peritoneum and blood, was reproducible with the same and three other strains of pneumococcus type 3. Penicillin administration during the lag phase had no effect upon survival of the mice, while a significantly improved effect was obtained after treating the mice with penicillin during the bacterial growth phase. In contrast, treatment with gentamicin or erythromycin during the bacterial lag phase was significantly more effective than administration of either drug after initiation of growth of the pneumococcus.