Effect of adding cysteine to brain-heart infusion broth on the isolation of Bacteroides fragilis from experimental blood cultures.

Adding 0.05% cysteine to brain-heart infusion broth greatly improved the recovery of three strains of Bacteroides fragilis from simulated blood cultures. No bacteroides were isolated after 24 hours' incubation in brain-heart infusion broth without added cysteine, and this was therefore a poor medium for the isolation of B. fragilis. Results with brain-heart infusion broth containing 0.05% cysteine were similar to those obtained with commercially prepared Thiol and Brewer's thioglycollate media.