Single-celled microorganisms which can exist either as independent (free-living) organisms or as parasites (dependent upon another organism for life).
Examples of bacteria include:
* Acidophilus, a normal inhabitant of yogurt,
* Chlamydia, which causes an infection very similar to gonorrhea,
* Clostridium welchii the most common cause of the dreaded gas gangrene,
* E. coli, the common peaceful citizen of our colon and, upon occasion, a dangerous agent of disease, and
* Streptococcus, the bacterium that causes the important infection of the throat strep throat.
The term bacteria was devised in the 19th century by the German botanist Ferdinand Cohn (1828-98) who based it on the Greek bakterion meaning a small rod or staff. In 1853, Cohn categorised bacteria as one of three types of microorganisms -- bacteria (short rods), bacilli (longer rods), and spirilla (spiral forms). The term bacteria was preceded in the 17th century by the microscopic animalcules described by Antony van Leeuwenhoek (1632-1723).
