A Novel Selective Medium for Isolation of Bacteroides fragilis from Clinical Specimens

A novelBacteroides fragilis selective (BFS) medium, consisting of a brain heart infusion agar base supplemented with yeast extract, cysteine hydrochloride, bile salts, vitamin K, hemin, glucose, esculin, ferric ammonium citrate, bromothymol blue, gentamicin, kanamycin, and novobiocin, was evaluated. When BFS agar was tested with a collection of 303 bacteria of different genera, it allowed the growth ofB. fragilis as large yellow colonies, with blackening of the medium after 48 h of anaerobic incubation, while the growth of most other anaerobes, facultative anaerobes, and aerobes was inhibited. In a prospective comparison of BFS agar with a routinely used medium (neomycin blood agar) in 1,209 clinical specimens, 60B. fragilis bacteria were detected on BFS agar while 46 were detected on the routine agar (McNemar's test,P = 0.008). In conclusion, this novel medium may be added to improve the recovery ofB. fragilis in clinical specimens and to facilitate surveillance of antimicrobial-resistant strains.