LEVELS OF CLASSIFICATION- Dr C R Meera

Taxonomy deals with identification, Classification and Nomenclature of organisms. Objective of taxonomy is to arrange organisms into categories that reflect the similarities of the individuals within the groups.

•   Carolus Linnaeus: 1700’s: Two Kingdoms: Plants and Animals

•   Carolus Linnaeus: 1753’s: Binomial nomenclature --"father of modern taxonomy"

•   Ernst Haekel: 1866: Kingdom Protista

•   R.H. Whittaker: 1969: Five Kingdoms: Monera, Protista, Fungi, Plantae and Animalia

•   Carl Woese: 1990: Three Domains:  archaea, bacteria, and eukaryote domains.

Taxonomic Ranks

 Th.e classication of microbes involves placing them within hierarchial taxonomic levels. Microbes m each level or rank share a common set of specific features. The levels are arranged in a non-overlapping manner so that each level includes not only the traits that define not only the rank above it, but a new set of more restrictive traits. The highest rank is the domain. All prokaryotes belong to the domain Bacteria or the Archaea. Within  each domain, each microbe is assigned to a  phylum, class, order, family , genus and species.  Some prokar·yotes  are also divided into subspecies.                                                                           An example of Taxonomic ranks and name is given below.

Rank	Example
Domain	Bacteria
Phylum	Proteobacteria
Class	Gammaproteobacteria
Order	Enterobacteriales
Family	Enrerobacteriaceae
Genus	Shigella
Species	S. dysenteriae

Order- ends with “ales” and Family with “ceae”

The basic taxonomic group (taxon) is species. Species of higher organisms are groups of potentially interbreeding natural populations that are reproductively isol ated from other groups. This definition is satisfactory for sexually reproducing organisms, but fails wjth many microorganisms as they do not reproduce sexually. So a prokaryotic species is a collection of strains having similar characteristics. In other words, prokaryotic species·is a collection of strains that share many stable properties and differ significantly from other groups of strains. Perhaps a species should be the collection of organisms that share the same sequences in their core housekeeping genes (genes coding for products that are required by all cells and which are usually continually expressed).

A strain is the descendants of a single, pure microbial culture. Bacterial species consist of a special strain called "type strain" together with all other strains that are considered sufficiently similar to the type strain. The type strain is the strain that is designated to be the permanent reference specimen for the species. It is the strain to which all other strains must be compared to see if they resemble it closely enough to belong to the species. So they are special and hence much care is taken to preserve and maintain them by national reference collections such as the ATCC (American Type Culture Collection) in United States or the National Collection of Type Cultures in England.

There are a number of different ways in which strains within a species may be described. Biovars are variant strains characterized by biochemical or physiological differences, morphovars differ morphologically, and serovars  have different antigenic properties.

 Just as bacterial species is composed of a collection of similar strains, a bacterial genus is composed of a collection of similar species. One of the species is designated as the type species and this serves as the permanent example of the genus.

Species	A group of similar Strains
Genus	A group of similar Species
Family	A group of similar Genera
Order	A group of similar Families
Class	A group of similar Orders
Phylum (Division)	A group of similar Classes
Domain (Kingdom)	A group of similar Phylum

Microorganisms are named by using the binomial system introduced by Carl Linnaeus. The Latinized, italized name consists of two parts. The first part, which is capitalized, is the generic name, and the second part is the uncapitalized species name (Eg. Escherichia  coli).