All bacteria come under the kingdom Monera. These are found everywhere. They have the highest number among microorganisms. They are also found in adverse habitats like hot water springs, ice, deep sea and desert etc. Many bacteria live as parasites on or inside other organisms.
Bacteria have been divided into four groups on the basis of their shape (spherical Coccus, rod shaped - Bacillus, comma shaped, Vibrium and spiral Spirillum). These are simple in structure. They can synthesise their food from inorganic substances. They are photosynthetic or chemosynthetic autotrophs, that is, they do not synthesise their own food but depend on other living organisms or dead organic matter for food.