Modified broth-disk method for testing the antibiotic susceptibility of anaerobic bacteria

The most commonly used method for testing the antibiotic susceptibility of aerobic and facultative bacteria is the disk diffusion method. However, some anaerobic bacteria do not grow well enough in anaerobic jars for performance of disk diffusion tests. A modification of the broth-disk method of Schneierson allowed us to determine antibiotic susceptibility in a completely anaerobic environment. Commercial antibiotic disks were added anaerobically to tubes of prereduced brain heart infusion broth to achieve a concentration of each antibiotic approximating that attainable in blood. The tubes were then inoculated and incubated for 18 h. Resistance or susceptibility to each antibiotic was determined according to the amount of growth in each tube as compared with a control culture without the antibiotic. There was good correlation between results obtained by this broth-disk method and minimal inhibitory concentrations.