The human microbiota – friend or fiend?

Océane Sadones

Early Stage Researcher 3, Ludwig Maximilian University of Munich

AdobeStock_microbiota.jpeg

The human microbiota is defined as an ecological community of microorganisms that can be found on or in a human being. What does this actually mean?

Yes, human bodies are covered by microorganisms – bacteria, fungi, viruses, and other friendly living organisms that we cannot see with our eyes. These microorganisms are also present in human bodies. They can be found anywhere – from the hair to the feet, covering the skin, or even from the oral cavity to the gut. In fact, the number of bacterial cells would be equal or even higher than our own human cells (Kho & Lal, 2018).

Microbiota can be differentiated based on the following:

-          Body parts: The bacterial species found in the gut are partially different from the ones found in the cervix, or at least present in contrasting ratio from one another.

-          Different individuals: Indeed, every one of us shows different compositions in terms of species present in the microbiota and the ratio between them. This leads to the formation of specific patterns that can be defined as “fingerprints” of each human being.

Although these differences would be found in healthy individuals living a similar lifestyle at the very same geographic place, it is also important to note that these distinctions between human beings have been demonstrated to be more important between people having different lifestyles and living in different places on Earth.

-          Timespan: Microbial fingerprints evolve during our lifetime and can be affected by many different factors such as antibiotic intakes, diet, etc.

The gut microbiota can be defined as an organ, the latest ever discovered in the human body. It is also the largest and most diverse human microbiota. As we all know, every human organ has a specific function. In addition, such a big quantity of microorganisms cannot be present in each of our guts for nothing. So, what is the function of the gut microbiota?

A first function is related to metabolism. Indeed, the bacteria present in our gut help us every day to increase our intake of nutrients. These bacteria contain machineries that degrade several components, which our body cannot assimilate, into other components that can be used as nutrients for us. This is one of the reasons why people from different parts of the world do not have the same pattern of microorganisms in their gut microbiota. Daily, you are feeding your bacteria food that is different from the one found elsewhere on the Globe. Therefore, the bacteria able to degrade your type of food are overrepresented among your microbiota. This explains also why you can feel slightly sick when you travel in foreign countries, as you do not have a microbiota that is adapted to the food you are eating.

We call the relationship between us and our bacteria a “symbiosis”. Indeed, a symbiotic relationship is defined as a close relationship between two species in which at least one benefits. In this case, both of us get to do it. The bacteria benefit from having a home and food, and we get to assimilate more nutrients thanks to our microbiota. Some studies using mice with and without a gut microbiota have shown that the animals lacking the microorganisms needed to eat much more to sustain a standard body weight.

A second function is antimicrobial protection. As you all know, microorganisms can also be pathogenic and cause diseases, even though we just saw that most microorganisms can be consider as “good” for us. The colonisation of our guts by the “good bacteria” prevents the “bad” ones from developing and cause diseases. This phenomenon is called “competitive exclusion”. The healthy microbiota prevents the development of infection by competing with pathogens for the environment. On the other hand, the microorganisms that constitute our gut microflora also train our immune system. Indeed, our defenses are challenged to be tolerant for beneficial bacteria and yet prevent pathogens to overgrow.

As mentioned before, some factors can affect the composition of the normal gut microbiota.

The age of an individual plays an important role in the variation of the gut microbiota composition. It has been shown that children, adults, and the elderly show great variation in their “fingerprints”.

The method of birth greatly affects the composition of the gut microbiota. Infants born vaginally usually have a gut microbiota composed of bacteria from the mother’s birth canal, while it’s the skin bacteria of the mother that shape the gut microbiota of infants born by Cesarean delivery.

Diet also has a key role in building a diversified microbiota. As an example, children fed with breast milk or formula feeds show different patterns of gut microbiota. At later stages, the diet is the most important factor that influences the composition of the gastrointestinal microflora. Studies have shown that the richness and diversity of a healthy microbiota come from a diet abundant in fibers, vegetables and fruits.

The intake of antibiotics has short and long-term implications in the differential composition found in the gut microflora. Antibiotics, particularly the commonly used large spectrum antibiotics, target not only the pathogen causing the disease but also our beneficial bacteria. This explains why several people develop mycosis after being treated with antibiotics. The equilibrium of the ecosystem is damaged, some microorganisms can overgrow and cause this side effect. The good bacteria are not playing their role to cause competitive exclusion and opportunistic microorganisms do not miss this chance to invade our body. Probiotics can be used to counteract this unwanted colonisation.

All these factors explain why people from different parts of the globe show different patterns in the composition of the gut microbiota (Jandhyala et al., 2015).

However, even though a large panel of gut microbiota exists, and differences in the diversity and richness of such microbiota are always found in healthy human beings, some patterns can be related to diseases. Obesity, autism, type-1, and type-2 diabetes, depression, and far more diseases are associated with the microbiota. Although none of us knows yet if the differences in the composition of the gut microbiota are a cause or a consequence of the disease, connections can be found (Leviatan & Segal, 2020). Healthy microbiota can even be used as a treatment for certain diseases (Neish, 2009).

Is that not amazing? Using bacteria to treat diseases?

REFERENCES:

Jandhyala, S. M., Talukdar, R., Subramanyam, C., Vuyyuru, H., Sasikala, M., & Reddy, D. N. (2015). Role of the normal gut microbiota. World Journal of Gastroenterology, 21(29), 8836–8847. https://doi.org/10.3748/wjg.v21.i29.8787.

Kho, Z. Y., & Lal, S. K. (2018). The human gut microbiome - A potential controller of wellness and disease. Frontiers in Microbiology, 9(AUG), 1–23. https://doi.org/10.3389/fmicb.2018.01835.

Leviatan, S., & Segal, E. (2020). Identifying gut microbes that affect human health. Nature. http://www.nature.com/articles/d41586-020-03069-8.

Neish, A. S. (2009). Microbes in gastrointestinal Health and Disease. Gastroenterology 136(1), 65-80. https://doi.org/10.1053/j.gastro.2008.10.080.

 
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