The bactericidal activity of ampicillin, amoxicillin, tetracycline, and doxycycline against H. influenzae under various conditions of culture.

The bactericidal activities of ampicillin, amoxicillin, tetracycline, and doxycycline at a concentration of 1 mg/l were compared for 13 strains of H. influenzae with a typical minimal inhibitory concentration of these drugs. Conditions of culture used were: liquid medium in shaken flasks, stationary flasks and stationary tubes, with and without the addition of 30% human serum. Ampicillin and amoxicillin exerted the same bactericidal activity. Survival rates at 5 h were on the average near 1%, however, with large variations. At 24 h survival rate was uniformly below 0.12%. The bactericidal activity of the tetracyclines depended more on the cultural conditions. A marked reduction of the bacterial population below 1% was found at 24 h between 12 of 12 and 5 of 13 strains according to the experimental circumstances. Tetracycline was more bactericidal than doxycycline, especially in the presence of serum.