What bacteria live in our gut?

Lactobacilli, bifidobacteria, staphylococcus.. what are they?

Our gut microbiota is composed of different species of bacteria, all competing for resources: food and “living space”. Some of these bacteria produce metabolic products that actually “help” our body, others can cause damage.

Even in healthy people, “good bacteria” do not comprise the totality of the microbes living in our gut. Furthermore, different parts of our digestive system contain different bacteria, as the environmental conditions vary quite dramatically.

The stomach consists of an environment which is very unfavourable to foreign organisms, this is due to its acidity and the presence of digestive enzymes. Despite that, it hosts a large community of different bacteria. Some of them were mentioned in the previous videos: the phyla Proteobacteria, Firmicutes, Actinobacteria and Bacteroidetes. These phyla include hundreds of different species that can be found in the gut, and there is a stark spatial inhomogeneity of their distribution.

Furthermore, the stomach microbiota is not static: every day we ingest more than ten billion new bacteria, that might or might not belong to the same species of the ones that already coexist with us.

However, one of the bacteria that is commonly found in the stomach has a name that is well known even outside of the researcher community: Helicobacter pylori. If being host to this bacterium could be considered an illness, then almost half of the world would be ill! But can it be considered only as a pathogenic bacterium?

The answer is maybe. This bacterium is contagious, and it is linked with many upper gastrointestinal tract pathologies, such as gastritis and peptic ulcers, and its presence is linked with an increased risk of cancers. However, having this bacterium in our stomach is not an illness in itself, and in many people, it finds a niche environment in the stomach without causing any pathology. In fact, testing for the presence of this bacterium is not routinely done, and therapies to eliminate it are usually taken only when other symptoms develop.

Furthermore, some scholar such as Martin Blaser, director of the Human Microbiome Program at the New York University, believe that Helicobacter pylori could actually play a useful role in for instance by helping to regulate the stomach acidity, and that its eradication could worsen conditions such as gastroesophageal reflux disease.

What about the large and small intestine?

The small intestine contains a relatively less diverse microbial population, and the diversity seems to increase as we move further toward the lower digestive system. We can find Bacteroidetes here, as well as other bacteria that don’t use oxygen – anaerobes. Some of these bacteria include Streptococcus, Clostridium and Enterobacteria.

In the large intestine, we can instead find a diverse microbiota, consisting of thousands of different bacteria. As in the previous parts of the gastrointestinal tract, we can find Bacteroidetes, as well as Firmicutes.

What about other bacteria, that are known for their pathogenic properties or because they are contained in probiotic supplements? Lactobacilli, bifidobacteria and a known pathogen, Staphylococcus aureus, are present in relatively large concentrations.

Another bacterium that can be found in the gut and is often linked to food contaminations and recalls is Escherichia Coli. However, not all strains of this bacterium cause gastrointestinal symptoms: many strains are harmless, and they can actually help us, by synthesizing vitamins, like vitamin K2, that we then can absorb and use.

Knowing the name of a bacterial phylum or genera contained in a food or in a supplement often is not enough to determine whether it will be helpful to us, or whether it will even survive in our digestive system.
Studies usually specify the exact strain of bacteria that are found in a certain group of subjects or are used in a certain intervention.

Given the large variability found in our stomach, drawing overly broad generalizations can mislead us, and lead us toward foods, or supplements, whose effectiveness is unproven.