Bacteria are ten to 100 times larger than viruses. They are typically 1 to 3 microns in length and take the shape of a sphere or rod. Most bacteria consist of a ring of DNA surrounded by cellular machinery, all contained within a fatty membrane. They acquire energy from the same essential sources as humans, including sugars, proteins, and fats. Some bacteria live and multiply in the environment while others are adapted to life within human or animal hosts. Some bacteria can double in number every fifteen minutes while others take weeks or months to multiply. Bacteria cause many types of diseases, ranging from mild skin irritation to lethal pneumonia.
Bacteria are prokaryotic organisms (Kingdom:Monera), without cell defined organelles like mitochondria,Golgi bodies,Endoplasmic reticulum.,etc Microscopic, unicellular, they may occur singly or aggregations to form colonies. They posses rigid cell wall. Cell wall is made up of peptidoglycan (Mureins) and Lipo polysaccharides. Absence of well defined nucleus.i.e., DNA is not enclosed in a nuclear membrane. Ribosomes are scattered in the cytoplasmic matrix and are of 70S type. The plasma membrane is invaginated to form mesosomes. Most of the bacteria are heterotrophic.Some bacteria are autotrophic, posses bacteriochlorophyll, which is not in plastids.Instead it is found scattered. Motile bacteria posses one or more flagella. The common method of multiplication is binary fission. True sexual reproduction is lacking,but genetic recombination occurs by conjugation ,transformation and transduction.