Bacteria are single-cell 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 the colon and, upon occasion, a dangerous agent of disease, and Streptococcus, the bacterium that causes the important infection of the throat, strep throat. These are only a few of the thousands of known species of bacteria. “Bacterium” is the singular and “bacteria” the plural — one bacterium, two or more bacteria. The term was devised in 1847 by the German botanist Ferdinand Cohn who based it on the Greek “bakterion,” a small rod or staff reflecting the shape of the first observed bacteria.