The scientific study of organisms is the ultimate objective of characterizing and arranging them in orderly manner. Since microbial world shows extraordinary diversity, it is important to classify and arrange them into groups based on mutual relationships.
The biological classification, in broader sense, consists of three separate but interrelated parts: classification, nomenclature and identification.
Classification is the arrangement of organisms into groups or taxi depending on their alikeness or evolutionary relatedness. Nomenclature is concerned with assignment of names to taxonomic groups as per accepted norms. Identification is the process determining the particular organism to specific tax.
In microbiology, tax initially starts from the strains. A strain is made up of all descendents of a pure culture of particular microorganism. It is usually a succession of cultures derived from an initial colony. Each colony has a specific history and designation.
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The basic taxonomic group is called the species. It is a collection of strains having similar characteristics. Definition of species is different to that applied to higher organisms.
Species of higher organisms are groups of interbreeding or potentially interbreeding natural populations that are reproductively segregated from other groups. However, microbes are different because many of them rarely reproduce sexually and hence, it is difficult to find out their phenotypic and genotypic differences.
Hence the species of bacteria (a kind of microbe), for example, can be defined as a group of similar strains distinguished sufficiently from other similar groups of strains by genotypic, phenotypic and ecological characteristics. It has been accepted that a bacterial species would include strains with 70 per cent or more of DNA-DNA relation and with 5 per cent or less in thermal stability.
A genus is well-defined group of one or more species that is clearly separate from other genera. In practice, there are many points in assigning microbes to higher ranks.
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Family:
A group of similar genera, Order: – A group of similar families, Class:- A group of similar Orders, Division:- A group of similar classes, Kingdom :- A group of similar divisions
The microbial taxonomists focus on two points to make classifications successful and acceptable one.
Stability:
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When classifications are subject to frequent, radical changes, then, it leads to confusions. Therefore, it should be devised in such a way that it can accommodate minor changes when new information is available.
Predictability:
By knowing the characteristics of one member of a taxonomic group, it should be possible to assume that the other members of the same groan I have similar characteristics. If this cannot be done, then the classification loses it significance.