Medium for selective isolation and presumptive identification of the Bacteroides fragilis group

Abstract

We developed and evaluated a new medium (FRAG agar) for the selective isolation and presumptive identification of the Bacteroides fragilis group. This medium contains 1% D-glucuronic acid as a fermentable carbon source, a reduced peptone content, gentamicin, and 20% bile. Presumptive identification of the B. fragilis group was based on growth, fermentation, and typical colony morphology. A total of 75 stock culture isolates of the B. fragilis group grew well on this medium, and 69 showed evidence of fermentation. Of 90 other anaerobes, none grew well or fermented glucuronic acid. In a clinical trial of 100 specimens sent for anaerobic culture, FRAG agar inhibited 71 of 71 anaerobes not belonging to the B. fragilis group, as well as 104 of 110 facultative organisms. A total of 33 isolates of the B. fragilis group were recovered on the selective medium, whereas only 23 were recovered by routine methods. Of 23 cultures positive for the B. fragilis group on routine plates, 22 were positive on FRAG agar.