Bacteria are unicellular micro-organisms whose size is measured in micrometres. Their genetic information is contained in a liquid called cytoplasm which is surrounded by a membrane and a wall. The wall is an essential part of the bacterium. There are two types of walls: thick and dense, and thin and soft. This difference led to the classification of bacteria into two major groups: the thick-walled Gram-positive (Gram is a bacterial pigmentation technique), and the thin-walled Gram-negative.
It is impossible to know precisely how many varieties of bacteria there are. Many are probably still unknown. They may be found in all mediums and most have no effect on man. However, some do have harmful effects, such as the pneumococci (we use the common names here and in what follows) which may cause pneumonias, gonorrheas (certain sexually transmitted diseases), Koch's bacillus (tuberculosis), Hansen's bacillus (leprosy), and those responsible for tetanus, botulism, gangrene and other infections. This does not mean, however, that all bacteria are harmful. One needs only think of lactobacillus that is found in yogurts, or antibiotics that many of them produce.