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Name: Anisah O.
Status: Other
Age: 30s
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Question:
Except for the shape, are there other ways by which different bacteria can be classified?



Replies:
Morphology, that is the shape of bacteria, is still an important tool for the identification of species. Morphology in combination with specific staining procedures (Gram stain for example) is routinely used for identification. However that is not sufficient for classification: definingh which species are related.

Modern classification of bacteria is heavily dependent on the DNA sequence of a particular component of their ribosomes. Ribosomes are the particles that produce protein from RNA. They consist of protein and RNA themselves, and their RNA is strongly conserved between species. Since they evolve slower than other genes they are used as a tool to group bacteria together that are believed to have evolved from a common ancester. Their ribosomal RNA would be very similar. The sequence of this socalled 16S RNA is used to generate phylogenetic trees. These should represent the evolution of related species from common ancesters.

Other than this, rather fancy, genetic tool, bacteria are classified according to their total DNA content, which can have more G+C than A+T, or the other way round. The G+C content is relatively conserved between related species. This is combined with biochemical properties and growth characteristics. The latter is often used to subdivide related bacteria into species or sub-species. They can be measured by growing bacteria on specific media, or by performing specific biochemical reactions on them. That is the classical bacteriology that is routinely performed in bacteriological laboratories.

Frequently, bacteria are isolated that do not fit in the schemes of species that we have. Novel species are frequently discovered. Less frequently, novel genera are described. These are usually not newly discovered bacteria (the discovery of a completely novel genus is the exception), but rather existing genera and species that, on the basis of their ribosomal RNA or other characteristics, are now believed to belong to a different genus than they were ascribed to. The result is that they get a new name, and this renaming (as a consequence of reclassification) of bacteria can be quite confusing.

Trudy Wassenaar


In addition to individual shape bacteria can be classified by:

1. colony shape in culture
2. motility
3. morphological characteristics other than shape...eg multiple flagella
4. metabolic activity...eg. What sugars do they ferment
5. and DNA sequence

Peter Faletra Ph.D.
Office of Science
Department of Energy



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