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Understand  –  Bacteria

Essential for life

Microbes permeate not just the entire planet but are also found in large numbers in animals and humans. The first thing to learn about bacteria is that most are beneficial and do not cause disease. In fact, they play a vital role - in the environment, animals, and humans.

The human microbiota

An adult human is colonized with many hundreds of bacterial species, and the total microbial biomass in an average adult is around 0.2 kg. Bacteria and other microbes (like fungi and viruses) in the body make up the human microbiota. The majority is located in the gastrointestinal tract, but all surfaces in contact with the environment are colonized with bacteria, such as the skin, upper respiratory tract and genital tract. The microbiota co-exist with the human host and have many important functions, as described in this video.

Most bacteria are good for us

The bacteria in our bodies help degrade the food we eat, help make nutrients available to us and neutralize toxins, to name a few examples. Also, they play an essential role in the defense against infections by protecting colonized surfaces from invading pathogens.

When talking about both microbes and their genomes (DNA), the term microbiome is often used. Recently there has been an increase in studies of the human microbiome. It is becoming more and more evident that microbes are important for human health, but also disease development. Inflammatory bowel disease, gastric ulcers, colonic cancer, and obesity are examples of conditions for which the composition of the microbiota has been indicated to play a role.

Other microbiomes

Apart from humans, microbiomes are also found in animals and in different habitats on Earth. Animals, like humans, have microbiomes that are essential for their lives and functions. The most influential bacteria for life on Earth are found in the soil, sediments, and seas. The well-known functions of these are to provide nutrients like nitrogen and phosphorus to plants as well as to produce growth hormones. By decomposing dead organic matter, they contribute to soil structure and the cycles of nature.

Effects of Antibiotics on the Microbiome

All antibiotics affect and kill pathogens at the site of infection but also bacteria that normally live in the body. This can cause several unwanted side effects and diseases that should prevent us from taking antibiotics when not needed. For example:

  • Decreased diversity

Exposure to antibiotics can lead to changes and/or destabilization (dysbiosis) in the human microbiota. This can in turn lead to diseases not necessarily directly caused by a single pathogen but caused by disruption of the composition and diversity of the microbiota.

  • Selection of resistant bacteria

Whenever a population of bacteria is exposed to an antibiotic, any bacteria that are resistant to that antibiotic gain an advantage. As many other bacteria die, the resistant ones can increase in number. Widespread use of antibiotics thus leads to the selection of resistant bacteria that in turn may spread and cause infections that are difficult to treat.

Learn more in “Mutations and selection” and “Why should I care”.

Selected Resources

Resource Description
All you wanted to know about microbes but were afraid to ask… – The human microbiome (PDF 0,34MB) Factsheet from ReAct about the microbiome, and the effects antibiotics have on it. Also available in Spanish (El microbioma humano, PDF 0,38MB).
Your Microbial Friends Information portal with interactive graphics to explore the human microbiome (Genetic Science Learning Center, University of Utah).
How Bacteria Rule Over Your Body – The Microbiome Video. Short animated movie about the microbiome (by Kurzgesagt) (8 min).
Antibiotics and the human microbiome Video about antibiotics and their impact on the human microbiome by Institute for Systems Biology (ISB), expert comments by Assistant Professor and microbiome researcher Dr. Sean Gibbons (5 min).

More from "Bacteria"

Institute for Systems Biology. Antibiotics and the human microbiome. (2019).
Kurzgesagt – In a Nutshell. How Bacteria Rule Over Your Body – The Microbiome. (2017).
Berg, G. et al. Microbiome definition re-visited: old concepts and new challenges. Microbiome 8, 103 (2020).
Lozupone, C. A., Stombaugh, J. I., Gordon, J. I., Jansson, J. K. & Knight, R. Diversity, stability and resilience of the human gut microbiota. Nature 489, 220–230 (2012).
Valdes, A. M., Walter, J., Segal, E. & Spector, T. D. Role of the gut microbiota in nutrition and health. BMJ 361, k2179 (2018).
Fan, Y. & Pedersen, O. Gut microbiota in human metabolic health and disease. Nat Rev Microbiol 19, 55–71 (2021).
Byrd, A. L., Belkaid, Y. & Segre, J. A. The human skin microbiome. Nat Rev Microbiol 16, 143–155 (2018).
Flint, H. J., Scott, K. P., Louis, P. & Duncan, S. H. The role of the gut microbiota in nutrition and health. Nat Rev Gastroenterol Hepatol 9, 577–589 (2012).
The HIDDEN World of Microbiomes - YouTube.
Sender, R., Fuchs, S. & Milo, R. Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLOS Biology 14, e1002533 (2016).
ReAct - Action on antibiotic resistance. All you wanted to know about microbes but were afraid to ask... - The human microbiome. (2017).
Bonfrate, L., Tack, J., Grattagliano, I., Cuomo, R. & Portincasa, P. Microbiota in health and irritable bowel syndrome: current knowledge, perspectives and therapeutic options. Scandinavian Journal of Gastroenterology 48, 995–1009 (2013).
Cho, I. & Blaser, M. J. The human microbiome: at the interface of health and disease. Nat. Rev. Genet. 13, 260–270 (2012).
University of Utah Health Sciences. Your Microbial Friends.