In this study, we found that the bacterial load in the gastric mucosa was determined to be 6.9×108 per gram of tissue. It is much lower than the abundance of bacteria present in the intestine 25, indicating that the human stomach is relatively hostile to the bacterial colonization 26. Findings from the present study, however, indicated a markedly increased bacterial load in gastric cancer. Bacterial overgrowth in the stomach has been found in various precancerous conditions 13,27, including hypochlorhydria and mucosal atrophy. It has been suggested that microbes in the stomach are involved in the production of carcinogens and promotion of inflammatory injuries 15,28 Thus, bacterial overgrowth is a potential cancer-promoting factor 17. Nonetheless, further studies are indicated to clarify whether the bacterial overgrowth is a consequence of cancerous mucosa that generates environments favoring bacteria proliferation. Both Chao1-estimated richness and Shannon’s diversity index reflect the number of species in a microbial community 29. Our results found that they were similar between gastric cancer and chronic gastritis, indicating that there is no alteration in the number of bacterial species in the microbiota from gastric cancer. The PCoA analysis takes into account the bacteria phylogeny 29. In contrast to chronic gastritis, our results showed a scattered pattern in gastric cancer. This indicated that members of microbiota in gastric cancer were more distantly related, suggesting a diversified microbiota harbored in gastric cancer. Taken together, these results indicate bacterial overgrowth of diversified microbiota in gastric cancer. The contribution of such alterations toward the development of cancer requires further investigations.
Nitrate/nitrite and their metabolites are associated with a variety of functions. Acidified nitrite is capable of killing bacteria 32. Nitrate could shape the intestinal microbiota when acting as a source of energy 42. Nitric oxide, a final product of nitrite reduction, is intensively involved in the protection of mucosal integrity 43. Importantly, N-nitroso compounds derived from metabolisms of nitrate/nitrite are potent carcinogens 15,16. E. coli, Lactobacillus, and Nitrospirae are all known to play a role in the metabolisms of nitrate/nitrite 42,44,45. As the level of nitrate/nitrite increases in the gastric cancer and its precancerous conditions 46, it could be expected that the production of N-nitroso compounds is possibly enhanced by these bacteria. Thus, these enriched bacteria could participate in the carcinogenesis.
There are no conflicts of interest.
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