Antibiotic action on phagocytosed bacteria measured by a new method for determining viable bacteria

Antibiotics such as rifampin that act on phagocytosed bacteria have clinical advantages. The investigation of such intraphagocytic activity of antibiotics is, however, hampered by the time-consuming and cumbersome procedures necessary for enumerating viable bacteria. We have developed a semiautomatic biophotometric method for this purpose that permits the processing of relatively large numbers of samples. Using this method with Staphyloccus aureus Wood 46 as a test organism, we studied the dose dependence of the activities of a number of antibiotics against intraphagocytic bacteria. We could confirm the very good intracellular activity of rifampin. Its activity at low concentrations was slightly better against intracellular than against extracellular bacteria. In contrast, clindamycin and erythromycin, both of which have been reported to accumulate within phagocytes, did not exhibit a correspondingly enhanced intracellular activity, erythromycin being active intracellularly only at high concentrations. Clindamycin and vancomycin were effective against intracellular organisms, but only at relatively high concentrations owing to their low bactericidal activity against S. aureus Wood 46. Penicillin G, ampicillin, gentamicin, and streptomycin exhibited no useful intracellular activity. These results demonstrate that radioactive accumulation studies of antibacterial agents are not sufficient to show intracellular activity. Intracellular activity must be demonstrated in a functional test, preferably with graded concentrations of the test substance.