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