The Classification and Identification
of Bacteria of Medical Importance

The classification and identification of organisms are two separate but inter-related processes. Classification involves the identification of groups of organism that share common properties and that differ from other groups. Identification entails the assignation of an unknown organism to a group within a scheme of classification.

For more than a century bacteria have been classified according to their "Gram reaction" - named after Christian Gram who devised the protocol for his staining process in 1884. This is based upon their ability or otherwise to retain the a crystal violet-iodine complex when treated with organic solvents such as acetone or alcohol. Gram-positive bacteria retain the stain, and hence appear purple or blue-black when visualised by bright-field microscopy. In contrast, Gram-negative bacteria cannot retain the dye complex, and need to be counterstained with a red dye such as carbol fuchsin before they can be seen in the bright-field microscope.

As well as using their Gram reaction, bacteriologists also use the shape of bacteria to classify them. Bacteria display three basic shapes: round (cocci, from the Greek kokkos - a berry), rod shaped (bacilli, from the Latin bacillus - a stick or rod), or spiral.

Below are some notes on the classification of bacteria of medical importance, but you should be aware that bacteria capable of causing disease represent only a small minority of bacteria that inhabit the Planet. Furthermore, the source of an isolate often gives important clues as to its identity.