Selective medium for isolation of Bacteroides gracilis

Abstract

A new medium selective for Bacteroides gracilis was developed. The medium is tryptic soy agar (Difco Laboratories, Detroit, Mich.) containing nalidixic acid, teicoplanin, sodium formate, sodium fumarate, and potassium nitrate. Ail 18 strains of B. gracilis tested grew with only minimal inhibition. Most of the other 214 organisms tested, including most Bacteroides species, other anaerobes, and a substantial number of facultative anaerobes, were significantly inhibited by the medium. In a diagnostic study of 49 clinical specimens (28 patients with intra-abdominal infection, mostly gangrenous or perforated appendicitis), four strains of B. gracilis were isolated (from 4 different patients) on B. gracilis selective agar but were not detected on standard media.

Selective medium for Bacteroides gracilis

“…Rather than choosing tryptic soy agar as a basal medium, we used fastidious anaerobe agar, which has been shown to be excellent for the growth of fastidious anaerobic bacteria (2). Fastidious anaerobe agar is much more complex than tryptic soy agar, even though it does not incorporate potassium nitrate, a supplement added by Lee et al (3). However, to date there have been no studies which show enhancement of growth by nitrate in the presence of formate and fumarate.…”

“…A problem with the medium described by Lee et al (3) was the difficulty in distinguishing between B. gracilis and other organisms isolated from clinical specimens. This difficulty might be partly overcome by the incorporation of, for example, urea, which would make it possible to identify urease-producing organisms.…”