Bacteria Bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about half of the
Exogenous bacteria of this concept is "bacterial flora," which consists of exogenous bacteria ingested and endogenously colonized during the early stages of life. Bacteria that are part of normal internal ecosystems, also known as bacterial flora, are called "Endogenous Bacteria". A significant amount of prominent diseases are induced by "exogenous bacteria" such as "gonorrhea", "meningitis", "tetanus", and "syphilis." Pathogenic exogenous bacteria can enter a host via cutaneous transmission, inhalation, and consumption. Only a minority of bacteria species cause disease in humans; and many species colonize in the human body to create an ecosystem known as bacterial flora. Bacterial flora is endogenous bacteria,