Introduction: The term "microbiota" refers to all microorganisms present in a specific environment. For a long time, it was believed that the human stomach was devoid of any microorganisms due to the presence of gastric acid. However, this hypothesis was disproven with the discovery of the bacterium Helicobacter pylori. It was also found that other bacteria, as well as microorganisms like fungi and viruses, are present in the stomach's microbiota. Some of these stomach bacteria are beneficial to human health, while others are pathogenic. Among the diseases in which the stomach microbiota is involved is gastric cancer, where the stomach progresses from a normal condition to a precancerous state, such as gastritis, and eventually to gastric cancer. This progression is known as the Correa cascade. In this study, for the first time in Iran, the differences in the stomach microbiota of patients with atrophy and healthy individuals are examined using metagenomics methods.

Materials and Methods: This study utilized 10 biopsy samples, including five normal and five atrophic samples. After initial preparation, the samples were sent to a specialized company for DNA extraction and 16S rRNA metagenomic analysis. After receiving the results, the samples were assessed for quality and analyzed using CLC Workbench software. This analysis included examining the dominant bacterial population in each sample, evaluating bacterial abundance, and analyzing alpha and beta diversity. The results were also compared with studies conducted abroad and with results obtained from traditional culture method.

Results: In both normal and atrophic samples, the phylum Firmicutes was the dominant bacterial phylum. The predominant genera within this phylum included Staphylococcus, Enterococcus, and Streptococcus. In atrophic samples, the abundance of Firmicutes decreased compared to normal samples, while the phylum Proteobacteria increased. The phylum Firmicutes constituted a significant portion of the gastric microbiota in all foreign samples except those from Colombia. In all samples except those from China, microbial diversity in atrophic samples increased compared to normal samples. The normal Chinese samples had the highest diversity among the studies. Additionally, 28 bacteria were identified through culture-based study, with Firmicutes being the dominant phylum in this study as well.

Discussion: This study reveals significant differences in the gastric microbiota between atrophic and healthy individuals, highlighting a decrease in Firmicutes and an increase in Proteobacteria in atrophic samples. These findings contribute to our understanding of the microbiota's role in gastric health and disease, particularly in the context of the Correa cascade. Further research is essential to elucidate the implications of these microbial shifts for gastric cancer development and potential therapeutic interventions.