Arginase activity variation in H. pylori clinical isolates. Ornithine concentration was measured at 515 nm spectrophotometerically by the appearance of an orange color originating from the reaction of ornithine with acidified ninhydrin. A. Arginase activity of extracts from 16 US H. pylori strains, 11 of which are minimally-passaged clinical H. pylori isolates. Strains were grown on Campylobacter blood agar plates for 48 h. The graphs show the average arginase activity (pmol L-Orn/min/mg protein) ± standard deviation of one experiment representative of at least three. A complete list of strains and average arginase activities is given in Table S1 (see Additional file 1). The variation from experiment to experiment is about 10–15%. B. Comparison of arginase activity of H. pylori grown in broth versus grown on agar. H. pylori strains were grown in Ham's F-12 broth for 16–18 h or on Campylobacter blood agar plates for 48 h as described in Materials and Methods. The data suggest a strong correlation between the arginase activity from H. pylori grown in broth versus that on agar.