Microbiota of the Stomach and Small Intestine

The human gastrointestinal (GI) tract is characterized by distinct physicochemical conditions that shape its microbial communities. Among these, the stomach presents a particularly challenging environment for microbial colonization due to its highly acidic pH, ranging from 1 to 3. This extreme acidity effectively limits microbial density. However, certain acid-tolerant microorganisms are capable of surviving in this niche. Notably, Helicobacter pylori can colonize the gastric mucosa, particularly in the antrum, where it can persist despite the harsh conditions. Other microbial inhabitants detected in gastric fluids, although at low abundance, include members of the phyla Bacillota, Bacteroidota, Actinomycetota, and Pseudomonadota.

As gastric contents pass into the duodenum, the environment gradually shifts. The pH becomes less acidic, allowing a modest increase in microbial presence. This region is dominated by species from the Bacillota, Pseudomonadota, and Actinomycetota phyla. Nevertheless, microbial growth remains constrained due to the influx of bile and pancreatic secretions, which are rich in digestive enzymes and antimicrobial peptides. These secretions exert strong antimicrobial effects, reducing microbial proliferation.

The jejunum marks a transition toward a more hospitable environment for microbial colonization, with a progressive increase in microbial populations. This trend continues in the ileum, the terminal segment of the small intestine, where the pH becomes slightly alkaline. Here, the microbial community becomes more diverse and abundant, comprising facultative and obligate anaerobes. Common organisms include Bacteroides spp., Clostridia, and members of the Enterobacteriaceae family. These microbial populations play vital roles in nutrient metabolism and interact closely with the host immune system, underscoring the functional significance of microbial gradients along the small intestine.