Microbiota of the Stomach and Small Intestine

The stomach maintains a highly acidic pH of 1 to 3, creating a hostile environment that limits most microbial growth. However, acid-tolerant species like Helicobacter pylori can colonize the gastric mucosa, particularly in the antrum. Gastric fluids still contain members of Bacillota, Bacteroidota, Actinomycetota, and Pseudomonadota phyla, though at low abundance.

As gastric contents enter the duodenum, pH becomes less acidic, allowing modest microbial growth. The duodenum hosts a low-biomass community enriched in Bacillota, Pseudomonadota, and Actinomycetota. However, bile acids and pancreatic digestive enzymes constrain bacterial proliferation, maintaining relatively low microbial density compared to downstream regions.

Microbial populations increase progressively in the jejunum as environmental conditions become more favorable. In the ileum, pH becomes slightly alkaline, supporting diverse facultative and obligate anaerobes including Bacteroides, Clostridium, and Enterobacteriaceae. These microbiota play vital roles in nutrient metabolism and host immune interactions.

Despite the stomach's extreme acidity, gastric fluids contain members of four major bacterial phyla: Bacillota, Bacteroidota, Actinomycetota, and Pseudomonadota. These bacteria exist at low abundance, with acid-tolerant species like Helicobacter pylori being the most notable colonizers of the gastric mucosa.

Bile acids and pancreatic secretions rich in digestive enzymes and antimicrobial peptides exert strong antimicrobial effects in the duodenum. These secretions reduce bacterial proliferation and shape the microbial community composition, constraining growth despite the less acidic environment compared to the stomach.

The ileum hosts both facultative and obligate anaerobes adapted to its slightly alkaline pH. Common organisms include Bacteroides species, Clostridium species, and members of the Enterobacteriaceae family. These anaerobic communities are more diverse and abundant than in upstream small intestine regions.

pH progressively increases from the stomach's extreme acidity (1-3) through the duodenum to the ileum's slightly alkaline environment. This pH gradient enables increasing microbial colonization and diversity along the small intestine, supporting the establishment of complex anaerobic communities that support nutrient metabolism.