Microscopic living organisms (microorganisms), most often unicellular, of spherical forms (shells), elongated or rod-shaped forms (bacilli).
The study of bacteria is bacteriology or microbiology. Most bacteria are harmless or beneficial to the body, with an estimated 1010 individuals in the mouth. However, there are many pathogenic species that cause many infectious diseases, such as tuberculosis. Bacterial infections can be treated with antibiotics.