Rapid microassays for clindamycin and gentamicin when present together and the effect of pH and of each on the antibacterial activity of the other.

Gentamicin was measured in the presence of clindamycin by using as the assay organism a strain of Staphylococcus aureus that was resistant to clindamycin; clindamycin was measured in the presence of gentamicin by using Clostridium perfringens as the assay organism. Both assays were in agar diffusion systems, were rapid (2 to 4 h), and had errors of less than 10%. Neither antibiotic antagonized or synergized with the other against a variety of organisms. The pK of clindamycin was 7.3, and the pK of gentamicin was 8.2. Gentamicin was 100 or more times as active in alkaline medium against both gram-positive and gram-negative organisms, but clindamycin showed a striking pH effect on activity only with gram-positive organisms.