We characterized the gastric microbiota for the first time in children with and without H. pylori and observed that when H. pylori is present, it tends to dominate the microbial community. In the H. pylori-negative patients, there was more relative abundance of gammaproteobacteria, betaproteobacteria, bacteroidia and clostridia classes and a higher bacterial richness and diversity.

Microorganisms in humans form complex communities with important functions and differences in each part of the body. The stomach was considered to be a sterile organ until the discovery of Helicobacter pylori, but nowadays, it is possible to demonstrate that other microorganisms beyond H. pylori can colonize the gastric mucosa and that the diverse microbiota ecosystem of the stomach is different from the mouth and the esophagus, and also from the small intestine and large intestine. H. pylori seems to be the most important member of the gastric microbiota with the highest relative abundance when present, but when it is absent, the stomach has a diverse microbiota. Proteobacteria, Firmicutes, Actinobacteria, Bacteroidetes, and Fusobacteria are the most abundant phyla in both H. pylori-positive and H. pylori-negative patients. The gastric commensal flora may play some role in the H. pylori-associated carcinogenicity, and differences in the gastric microbiota composition of patients with gastric cancer, intestinal metaplasia, and chronic gastritis are described. The gastric microbiota changed gradually from non-atrophic gastritis to intestinal metaplasia, and to gastric cancer (type intestinal).

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