Commensalism is an interaction between organisms where one species benefits and the other species does not benefit but is not harmed. Many of the bacteria within the human body are commensal, particularly within the gut. These bacteria help make up the gut microbiota, a complex web of microorganisms that has massive impacts on the immune system, metabolism, and even brain function and behavior.

Lifestyle and diet changes can all impact the commensal bacteria populations, dramatically altering their effects on the body.

Gut Microbiota Composition

Thanks to advancements in testing methods, we have gained a large amount of information about gut bacteria in a relatively short amount of time. The microbiota in the stomach and small intestine are not very diverse, hosting only a few species. Meanwhile the colon houses between 200 and 1000 different species.

Together, only 30 to 40 species make up 99% of the bacteria in the gut microbiota. Because bacteria work together, the quantities of each species can change how the microbiome functions as a whole.

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Where the Bacteria Cones From

As fetuses, humans are relatively free of bacteria, thanks to our mothers’ immune systems and bodies. However, once the amniotic sac breaks, the fetus is exposed. During vaginal delivery, a dose of microbes enters from the birth canal. After a cesarean section, the baby receives bacteria from the skin of their carers.

Over the next six months to three years, more bacteria enter the newly-developed gut microbiome as toddlers place more things into their mouths. Throughout the rest of our lives, diet, age, and geographical location all impact the commensal bacteria populations.

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Commensal Gut Bacteria and the Immune System

By fully colonizing the space, using all available nutrients, and actively releasing powerful compounds called cytokines, the gut flora can protect against invading pathogens. Additionally, the linings within areas like the intestines develop alongside the commensal bacteria populations, allowing them to work together to block invaders.

However, disruptions in the gut microbiota can have wide-reaching consequences, including contributing to autoimmune disorders. Having a diverse, healthy microbiome of commensal bacteria is one of the keys to a well-functioning immune system.

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The Gut–Brain Axis

One of the biggest discoveries from the research of commensal gut bacteria is the connection between the gut and the brain, called the gut–brain axis. As an example of how the two are linked, an imbalance of gut bacteria can weaken the barrier protecting the brain. This can lead to brain inflammation and conditions like Parkinson’s or Alzheimer’s disease. Alternatively, if the brain is under immense stress, it may impact the vagus nerve—one of the main pathways of the gut–brain axis—which then causes conditions like irritable bowel syndrome.

Even issues like depression and anxiety have links to the gut and its commensal bacteria. The full scope of the gut–brain axis remains a mystery, though experts believe that a healthy gut could fight certain brain conditions.

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Commensal Bacteria and Obesity

Commensal bacteria within the gut may also impact obesity. In studies between lean and overweight twins, researchers discovered that the lean individuals tended to have much more diverse populations of gut bacteria. For one, they had more varieties of Bacteroidetes, which help break down starches and fibers for energy.

Animal studies revealed that adding bacteria from people with obesity to mice increased the animals’ weights, while bacteria from lean organisms lead to weight loss.

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Finding Ways to Boost Commensal Bacteria Populations

Modern research has made it clear that bacteria diversity is a core component of a healthy gut, meaning it is integral to overall health, as well. Findings also show that it is possible to adjust commensal bacteria populations—and even do so relatively quickly.

One simple method is to eat a variety of fruits and vegetables. For example, kale is an extremely healthful food, but eating only kale does not lead to a healthy gut microbiome. Combine foods like kale with other vegetables or fruits like tomatoes, peppers, and berries to improve bacteria diversity.

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The Importance of Probiotics

When looking to improve gut health, probiotics are at the center of the conversation. Probiotics are microorganisms—including commensal bacteria—that take up residence in the gut. They are available through supplements or in fermented foods like yogurt.

Though their degree of efficacy when introduced orally is still in question, researchers have linked these bacteria to all kinds of beneficial effects, but the most proven are the reduction of lactose intolerance and acute diarrhea.

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The Role of Prebiotics

Often confused for probiotics, prebiotics are powerful compounds in food that can boost the growth and activity levels of commensal bacteria and other gut microorganisms. The bacteria break down prebiotics through fermentation, creating short-chain fatty acids. These acids then impact functions like improving vitamin absorption, lowering blood pressure, and even improving mental health and mood.

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Eat More Resistant Starch

Of the prebiotics, one of the most powerful is resistant starch, a hard-to-digest carbohydrate. The small intestine is where the body digests most starches, with non-digestible portions moving forward to the large intestine. There, it becomes a fuel source for colonic bacteria, leading to a range of benefits, including reducing the risk of type 2 diabetes. Foods rich in resistant starch include bananas, oatmeal, and green peas.

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Not Commensal, but Mutualistic

Experts have known for many years that the majority of bacteria in and on the human body are not harmful. However, it is only recently that scientists have begun to understand exactly what these commensal bacteria do. Even now, the field is full of mystery and requires further research.

However, most new evidence points to the fact that these commensal bacteria may not be commensal at all, but are instead mutualistic, as both humans and the bacteria benefit. Ensuring the health of the bacteria also ensures the health of our bodies.

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