In the stomach of a gastric ulcer patient who had been administered an anti-acid, a gram-negative and
urease-negative bacillus similar in size to Helicobacter pylori was infected together with H. pylori. According
to biochemical test and 16S rRNA gene analysis, the urease-negative bacterium was identified as
Kingella denitrificans, a human nasopharyngeal commensal. In contrast to the standard strain of
K. denitrificans, the isolate showed catalase activity, did not produce acid from glucose, and exhibited acid
tolerance. Acid tolerance of H. pylori was increased by cocultivation with the K. denitrificans isolate, but
not with other isolates of K. denitrificans. Disruption of physiological and immunological niche by dysbiotic
colonization of bacterium may provide pathological attributes to human stomach. Collectively, a
careful administration of anti-acids to the elderly, especially those with atrophic gastritis, is necessary to
avoid repression of the gastric barrier to bacteria.