Quantitative study of the interaction between two antibiotics by agar diffusion.

In order to quantify the interaction between two antibiotics by agar diffusion, a fractional critical concentration (FCC) index, analogous to the fractional inhibitory concentration (FIC) index, was defined. Two pairs of filter paper strips, containing different concentrations of each of two antibiotics, were placed at right angles on an agar plate. After 2 h of diffusion the strips were removed and the plate was inoculated with bacteria. The FCC index was calculated after overnight incubation. Three combinations of antibiotics were studied: trimethoprim-sulphamethoxazole, tobramycin-cefazolin and vancomycin-rifampicin. Over 80% agreement was obtained between the FCC indices and the FIC indices obtained by the chequerboard microdilution technique. The novel diffusion method thus appears promising and warrants further evaluation.