Bacteria are a large group of prokaryotic microorganisms (cells lacking a nucleus, as opposed to eukaryotic ones, that have nucleus, and that form the human body) that were among the first life forms to appear on Earth. They are commonly found in most of the planet’s habitats and live in symbiotic (mutually beneficial), commensal (eating from the same dish) or parasitic (living in or on another organism and benefiting from it) relationships with plants, animals and humans. Many people believe that all bacteria are pathogens. Although some of them can be harmful, most of them are beneficial and necessary for remaining in good health.
Bacteria are 10 to 50 times smaller than human cells. In the human body there are an estimated 40 trillion microorganisms, including at least 1000 different known species of bacteria with more than 3 million genes—that is, 150 times more genes than humans have. Large bacterial communities live on the skin and in our bodily cavities. The largest community lives in our guts: the gut microbiota.