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Understand  –  Bacteria

Essential for life

The first thing to learn about bacteria is that most are beneficial and do not cause disease. They play essential roles in many environments, including the human body.

The microbiota

An adult human is colonized with many hundreds of bacterial species, and the total microbial biomass in an average adult is approximately 0.2 kg. Bacteria and other microorganisms 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, that is, 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, the microbiota play an essential role in the defense against infections by protecting the colonized surfaces from invading pathogens.

Recent years have seen an increase in the studies of microbes in the body and their genomes (DNA). It is becoming more and more evident that these microbes are important for human health, but also disease. 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.

Environmental microbiomes

Apart from the human microbiome, microbiomes are also found in for example animals, different habitats on Earth and even the Earth as a whole. 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. Well known functions of these are to provide nutrients like nitrogen and phosphorus to plants as well as producing growth hormones. By decomposing dead organic matter, they contribute to soil structure and the cycles of nature.

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).
The invisible universe of the human microbiome Video. Short animated movie about the microbiome (by npr) (5 min).
The human microbiome and what we do to it Video about the microbiome by NPS MedicineWise, expert comments by Prof. D. Relman, Stanford University (5 min).
Fine Reading: There Is No ‘Healthy’ Microbiome Opinion. Blog post discussing our unique microbiomes (original opinion by Ed Yong in the New York Times).

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The HIDDEN World of Microbiomes - YouTube [Internet]. [cited 2019 Aug 15]. Available from:
Sender R, Fuchs S, Milo R. Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLOS Biology [Internet]. 2016 Aug 19 [cited 2017 Nov 16];14(8):e1002533. Available from:
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 [Internet]. 2013 [cited 2014 Nov 11];48(9):995–1009. Available from:
The human microbiome and what we do to it [Internet]. 2012 [cited 2014 Nov 21]. Available from:
Schaechter M, Reguera G. Fine Reading: There Is No “Healthy” Microbiome [Internet]. Small things considered. 2014 [cited 2014 Nov 24]. Available from:
The Invisible Universe Of The Human Microbiome [Internet]. 2013 [cited 2014 Nov 21]. Available from:
Bäckhed F, Ley RE, Sonnenburg JL, Peterson DA, Gordon JI. Host-bacterial mutualism in the human intestine. Science [Internet]. 2005 Mar 25;307(5717):1915–20. Available from:
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Cho I, Blaser MJ. The human microbiome: at the interface of health and disease. Nat Rev Genet [Internet]. 2012 Apr;13(4):260–70. Available from:
University of Utah Health Sciences. Your Microbial Friends [Internet]. [cited 2014 Oct 9]. Available from: