The pneumococcus and the mouse protection test: inoculum, dosage and timing.

Intraperitoneal inoculation in mice of a S. pneumoniae type 3 in beef broth resulted in immediate growth in vivo as evidenced by bacterial counts in peritoneal washings and in blood. Treatment with penicillin 1 hour after inoculation reduced the bacterial counts in vivo; however, different doses of penicillin-G showed a similar effect as measured by bacterial counts, in spite of differences in their effect upon survival of the mice. Therefore, the effect of antibiotics in vivo in this model was better correlated with death/survival of the animals. For comparative purposes the ED50, i.e. the 50% effective dose, should be determined. The effect of cephalosporins, i.e. cefuroxime and cefotaxime, in this model highly depended upon timing of the antibiotic administration as related to inoculation.