In vitro susceptibility of anaerobes to co-trimoxazole

The minimal inhibitory concentrations (MIC) of sulfamethoxazole and trimethoprim against 144 strains of obligately anaerobic bacteria were determined on supplemented Diagnostic Sensitivity Test (Oxoid) agar or broth. 58% of the strains were susceptible to sulfamethoxazole alone (MIC less than or equal to 16 micrograms/ml), only 12% were susceptible to trimethoprim alone (less than or equal to 1 microgram/ml), and 85% were susceptible to the combination of sulfamethoxazole plus trimethoprim (less than or equal to 16 micrograms/ml) at a ratio of 19:1. All 45 strains of the Bacteroides fragilis group were susceptible to the combination. At a ratio of 1:1, 78% of the strains showed synergistic activity, at a ratio of 16:1 there were synergism in 54% of the strains. When the strains were tested by a modification of the broth-disc method in Diagnostic Sensitivity Test broth, there was over 98% correlation with the MIC. There was poor correlation between inhibition zone diameters by an agar diffusion method and MIC.