Stomach — 1,000 CFU/mL
Your stomach's low pH levels limit the growth of microbes. But even in this high-acid environment, some microorganisms survive. Actinomycetota tend to be the dominant microbes in the stomach, along with Streptococcus, Staphylococcus,
Peptostreptococcus and Lactobacillus.
Lactobacillus has been dubbed a "psychobiotic"18 because of its effects on mental health, particularly anxiety and depression. Lactobacillus bacteria — found in many fermented foods — helps dampen stress responses and prevent depression and anxiety, in part, by modulating your immune system.19
Small Intestine — 1,000 CFU/mL to 100 Million CFU/mL
The concentration of microbes in the small intestine gradually increases, starting with 1,000 CFU/mL in the duodenum — the first part of the small intestine — and rising to 100,000 CFU/mL in the second part, known as the jejunum. In the third part, the ileum, there are about 100 million CFU/mL.
Firmicutes are abundant in the small intestine, but actinomycetota, which help break down complex carbohydrates and produce vitamins B12 and K2, are another dominant species. They also help protect against pathogens.20
Bacteroides are also found in the small intestine. They also play a role in breaking down complex carbohydrates and producing vitamins, as well as helping to regulate the immune system.21
Large Intestine — Up to 1 Trillion CFU/mL
Bacteroides are the dominant microbe in your large intestine, the area of your body with the highest concentration of bacteria. Bifidobacteria are also present. The composition of these microbes, and those elsewhere in your gut, are linked to health conditions like obesity and inflammation.
As noted in a review published in Nutrients,