Protection of phagocytosed bacteria against antimicrobial agents

Antibiotic sensitive Staphylococcus aureus phagocytosed by human neutrophil granulocytes were exposed to penicillin G (benzyl penicillin), gentamicin or rifampicin in concentrations from 2.5 U/ml, 2.5 microgram/ml and 0.2 microgram/ml to 250 U/ml, 250 microgram/ml and 20 microgram/ml, respectively. The bacteria were protected from the antibacterial effects of penicillin G and gentamicin. Considerable numbers of phagocytosed bacteria remained viable after 24 hours exposure to antibiotic concentrations that killed more than 99% of extracellular bacteria in less than one hour. Killing of intracellular bacteria was more pronounced with rifampicin, indicating that the penetration of this drug into the phagocytic vacuole is superior to penicillin G and gentamicin.