In 1676 Anton Van Leeuwenhoek discovered the microbial world by his simple microscope. It was only after the invention of compound microscope by Hooke in 1820, that bacteria came to lime light. These very minute creatures were designated as “small microscopic species” or “ Infusorial animalcules”. Louis pasteur(1822-95 ) made a detailed study of bacteria and proposed germ theory of disease. Robert Koch, a german microbiologist, was the first scientist to prove the cause and effect relationship between microbes and animal diseases. Ehrenberg(1829) was the first to use the term bacterium. The branch of study that deals with bacteria is called Bacteriology. Bacteria are unicellular organisms and they are prokaryotic. i.e they do not have a membrane bound nucleus and membrane bound organelles.
Bacteria are omnipresent. They are found in all environments, where organic matter is present. They are found in air, water, soil and also in or on the bodies of plants and animals. Some of the bacteria live as commensals (eg. Escherichia coli in the human intestine) and some live as symbionts (eg. Rhizobium) in the root nodules of leguminous plants. Several of them cause diseases in plants, animals and human beings.
Bacteria are very small, most being approximately 0.5 to 1 micron in diameter and about 3 to 5 microns in length.