The influence of human granulocytes and monocytes on the antibacterial activity of cefetamet against Staphylococcus aureus.

The presence of human granulocytes and monocytes increased the activity of cefetamet against non-ingested Staphylococcus aureus. During ingestion of S. aureus by granulocytes, the activity of cefetamet was equal to that found in the absence of cells. Enhanced activity of cefetamet 30 mg/L was observed during ingestion of S. aureus by monocytes. The effect of cefetamet on S. aureus ingested by granulocytes was much less than that on bacteria in a liquid medium, the maximum growth inhibition (ERmax) being 0.15 and 0.90 log/h, respectively. The concentration at which 50% of the maximum effect was achieved (EC50) was 19.7 mg/L under both conditions. Cefetamet was also more active against S. aureus ingested by monocytes than against bacteria in medium (ERmax 0.97 and 0.77 log/h, respectively). The EC50 under both conditions was 9.7 mg/L. The granulocyte-associated concentration of cefetamet was about on third of that in medium, whereas the monocyte-associated concentration was about two thirds of that in medium. The low intracellular activity of cefetamet against S. aureus ingested by granulocytes is explained primarily by the minimal intracellular growth of bacteria.