Simplified Method for Antimicrobial Susceptibility Testing of Anaerobic Bacteria

A simple, abbreviated broth dilution test (tube test) utilizing a commercially available medium and inexpensive disposable materials, and which could be performed entirely in room air, was developed and used to test the susceptibility of 100 strains of anaerobic bacteria to clindamycin, chloramphenicol, ampicillin, and tetracycline. Results are reported in categories of susceptibility: susceptible to concentrations surpassed in vivo with usual dosage, susceptible to concentrations surpassed in vivo with high dosage, and resistant to concentrations achievable in vivo. Results are compared to minimal inhibitory concentrations which were determined simultaneously by using a microdilution method in an anaerobic glove box. Twenty strains of Bacteroides fragilis , 10 strains of Fusobacterium , 20 strains of Clostridium , 10 strains of gram-positive non-sporeforming bacilli, and 30 strains of cocci grew to visible turbidity within 1 day of incubation. Of the 360 antibiotic-organism combinations tested, 98% were in a susceptibility category that corresponded (within one concentration) to the actual minimal inhibitory concentration as determined by the microdilution method. After 2 days of incubation, growth was more abundant, but results often indicated inappropriate degrees of resistance. Variation in inoculum size had little effect on results. Ten strains of B. melaninogenicus did not grow enough for susceptibility to be categorized accurately. The tube test could be used in any clinical microbiology laboratory for a limited number of susceptibility tests on anaerobic bacteria other than B. melaninogenicus without preparation of special media or purchase of special equipment.