Concept of Microbial Species
The term species denotes a basic taxonomic group in taxonomy. The term species have different definitions for prokaryotes and for eukaryotes.
1). The meaning of the term species for higher organisms is a group of organisms that can interbreed or potentially interbreeding populations that have been reproductively isolated from other organisms. It is a satisfactory definition for all organisms that can reproduce sexually.
2). For prokaryotes, the species is characterized by phenotypic. genotypic and phylogenetic criteria. Now a days the species is more precisely defined by rRNA sequences. Two prokaryotes showing about 95% resemblance in 16S rRNA are placed in a single species.
A prokaryotic species mean "a collection of strains that share many stable properties and differ significantly from other groups of strains."
It should be noted that the bacteria reproduce only asexually. This means that the sexual reproduction does not occur at all. So, the bacterial species is defined on the basis of similarity in their members and not on the basis of reproduction.
Because of researches, genome sequence data are progressively increasing. Hence, the definition of species needs a revision. Accordingly a species will mean a collection of organisms having same sequence in their core housekeeping genes."
The housekeeping genes code for all those products that are always required by a cell and are always expressed.
Concept of Microbial Strain
When the two clones of a species differ genetically and express some difference in their character, they are called strains.
Diffferent strains arise due to mutation or genetic exchange in an initial species. Thousands of strains can arise due to mutation in a microbiology laboratory. The strains can be designated by capital letters followed by some number.
e.g. Escherichia coli strains K12 and M43
Proteus vulgaris strains OX2. OX19 and OXK.
There can be several strains. They are described in different ways. e.g.
1. Biovars : Variant strain with different biochemical and physiological character.
2. Morphovar : Morphological variants of a species.
3. Serovar : Strains having distinctive antigenic properties.
4. Phagovar : Strains differing in their resistance to lysis by bacteriophages.
Type strain and Type species
1. A strain, which was frst isolated and completely characterized, is called type strains. It may or may not completely represent a species.
2. The type strain of a species is called type species, The strains that resemble a type strain are included in a species. When an isolate is characterized as a new species or strain, it is deposited in the appropriate culture collection centers. e.g. ATCC.
