Direct isolation of atypical thermophilic Campylobacter species from human feces on selective agar medium

Abstract

Campylobacter upsaliensis is the name which has been proposed for a new group of thermophilic campylobacter strains which differ from C. jejuni and C. coli in having a negative or weak catalase reaction. Primary isolation of these strains from human feces has been achieved only use of filtration techniques. We report here direct isolation of strains corresponding to C. upsaliensis from stools of six children. The strains were isolated on a newly described campylobacter-selective medium. The strains were oxidase positive, hippurate negative, nitrate positive, negative for H2S in triple sugar iron, and susceptible to cephalothin (30-μg disk) and nalidixic acid (30-μg disk), and they grew at 37 and 43 °C, but not at 25 °C. The selective medium used was a blood-free, charcoal-based medium consisting of Columbia agar base, activated charcoal, cefoperazone (32 μg/ml), vancomycin (20 μg/ml), and cycloheximide (100 μg/ml). The medium supported the growth of the weakly reacting or catalase-negative strains, with colony counts equivalent to those obtained on antibiotic-free horse blood agar. These strains could not be isolated directly from stool on Skirrow medium, and colony counts confirmed that this medium could not support a low inoculum of these organisms. The clinical significance of these strains is unknown. We conclude that C. upsaliensis can be isolated directly from stool by using a selective medium, without the need for filtration.