The Species Concept in Microbiology

The number of species of bacteria and archaea is surprisingly small, despite their early evolution, genetic, and ecological diversity.

Learning Objective

Describe the concept of polyphasic species

Key Points

The differences in species concepts between the Bacteria and macro-organisms, the difficulties in growing/characterising in pure culture (a prerequisite to naming new species, vide supra), and extensive horizontal gene transfer blurring the distinction of species makes differentiation difficult.
The most commonly accepted definition is the polyphasic species definition, which takes into account both phenotypic and genetic differences.
A quicker diagnostic threshhold is to separate species as less than 70% DNA-DNA hybridization, which corresponds to less than 97% 16S DNA sequence identity.

Terms

DNA hybridization
Hybridization is the process of establishing a non-covalent, sequence-specific interaction between two or more complementary strands of nucleic acids into a single complex, which in the case of two strands is referred to as a duplex. Oligonucleotides, DNA, or RNA will bind to their complement under normal conditions, so two perfectly complementary strands will bind to each other readily.
bacteria
Bacteria constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most habitats on the planet.
species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring.

Full Text

Judging Species in an Asexual Context

Bacteria divide asexually and for the most part do not show regionalisms. In other words, "Everything is everywhere. " Accordingly, the concept of species which works best for animals, becomes entirely a matter of judgement.

Biological classification

The hierarchy of biological classification's eight major taxonomic ranks. A genus contains one or more species. Intermediate minor rankings are not shown.

The approximately 5000 species of bacteria and archaea constitute a surprisingly small number, considering their relatively early evolution, genetic diversity, and ability to reside in all ecosystems on Earth. The reason for this numerical peculiarity lies in the differences in species concepts between the bacteria and macro-organisms and in the difficulties in growing and characterizing in pure culture (a prerequisite to naming new species, vide supra). In addition, the extensive amount of horizontal gene transfer among microorganisms results in the blurring of the distinctions between species among microorganisms.

The most commonly accepted definition is the polyphasic species definition,which takes into account both phenotypic and genetic differences. However, a quicker diagnostic ad hoc threshhold to separate species is less than 70% DNA-DNA hybridization, which corresponds to less than 97% 16S DNA sequence identity. It has been noted that if this were applied to animal classification the order of Primates would be considered a single species.

The International Journal of Systematic Bacteriology/International Journal of Systematic and Evolutionary Microbiology (IJSB/IJSEM) is a peer-reviewed journal that acts as the official international forum for the publication of new prokaryotic taxa. If a species is published in a different peer review journal, the author can submit a request to IJSEM with the appropriate description. If the information is correct, the new species will be featured in the Validation List of IJSEM.

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