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The influence of subinhibitory concentrations of clindamycin on opsonization and phagocytosis of Staphylococcus aureus was studied. S.aureus was grown overnight in the presence or absence of one half or one quarter of the minimal inhibitory concentration (MIC) of clindamycin. Radioactively labeled S. aureus was opsonized for various periods of time in different concentrations of normal serum, heated antiserum and serum of patients with agammaglobulinaemia or C3 deficiency. Complement-as well as antibody-dependent phagocytosis of the antibiotic treated S. aureus was significantly enhanced, compared to phagocytosis of the untreated control. Killing experiments showed that clindamycin-treated S. aureus was also better killed by the granulocytes than untreated S. aureus. The mechanism of action is likely to be an increased susceptibility of clindamycin-treated bacteria to antibody- and complement-dependent phagocytosis.

The in-vitro inactivation of seven antimicrobial drugs by human faecal substance has been investigated. Nalidixic acid, colistin, neomycin, tobramycin, temocillin, trimethoprim, and aztreonam were separately mixed in graded concentrations with faecal suspensions prepared from faeces of eight healthy volunteers. Each was mixed separately with a sample of each of the eight suspensions. All seven antimicrobial drugs appeared to be rapidly biologically inactivated by intestinal contents in a dose-dependent fashion, although not all to the same extent. The results may explain why with some unabsorbed or minimally absorbed drugs higher oral doses are required to achieve eradication of Gram-negative bacilli from the digestive tract than others, when selective decontamination (SD) is attempted in clinical practice.