General methods of classifying bacteria

General techniques of classifying bacteria

Three techniques are used for arranging bacteria into taxa:

The Intuitive method: A microbiologists who is properly familiar with the characteristics of the organisms he or she has been studying for many years may decide that the organisms represent one or more species or genera. Problem with this method is that the characteristics of an organism that seem important to one person may not be so necessary to another, and different taxonomists may arrive at very different groupings. However, some classification schemes based on the intuitive method have proved to be quite helpful and acceptable.

Numerical taxonomy: In an effort to be more objective about grouping bacteria, a scientist may determine many characteristics (usually 100 to 200) for each strain studied, giving each characteristic equal weight. Then using a computer he or she calculates the % similarity (%S) of each strain to every other strain. For any two strains, this is: Where NS is the number of characteristics that are the same (positive or negative) for the two strains, and ND is the number of characteristics that are different. (The method is sometimes made rigorous by making NS equal to the number of positive characteristics that are the same for the two strains, since what microbiota can do may be more necessity than what they cannot do.) Those strains having a high %S to each other are placed into groups; those groups having a high %S to each other are in turn placed into big groups, and so on. The degree of similarity needed to rank a group as a species, genus, or another taxon is a matter of judgement on the part of the taxonomist. This technique of classification has great practical necessity as well as being relatively unbiased in its approach; it also yields classification that has a high degree of stability and predictability.

Genetic relationships

The third and tusually reliable method of classification are based on the degree of genetic relatedness between organisms. This technique is the most objectives of all and is based on the most important aspects of organisms, their hereditary material (DNA). In the 1960s the development of that branch of science known as molecular biology provided techniques by which the DNA of one organism could be compared with that of other organisms. At first only low level comparisons could be made, based mol %G+C values. It is true that two organisms of the same or similar species that are usually closely related will have very similar mol %G+C values, and it is also true that two organisms having quite a different mol %G+C values are not very closely related. However, it Is important to realize that organisms that are completely unrelated may have similar mol %G+C values. Therefore, much more wide accurate methods of comparison were needed- namely, methods by which the DNA molecules from various organisms could be compared with respect to the sequence of their component nucleotides. This series is the most fundamental characteristics of an organism. Modern techniques have now made it possible to make such a comparison. The basic principles be described briefly as follows:

1. DNA homology experimental test: The double-stranded DNA molecules from two different organisms are heated to convert them to single strands. The single strands from one organism are them mixed or made same with those from the other organism and allowed to cool. If the two organisms are closely related, heteroduplexes will form. In a different words, a strand from one organism will pair with a strand from the other organism. If the two organisms are not soo closely related, no heteroduplexes will form. This method is most useful at the species level of classification.
2. Ribosomal RNA homology experiments and ribosomal RNA oligonucleotide cataloging: Two organisms may notusually be so closely related as to give a high-level of DNA homology, yet they may still form some degree of relatedness. Ribosomes, the small granular-appearing structures within the cell which manufacture proteins, are composed of proteins and RNA. The ribosomal RNA (rRNA) is coded for only a small fraction of the DNA molecule, the rRNA cistrons. In all bacteria so far known , the nucleotide sequence of these rRNA genes has been found to be highly conserved; that is, during evolution, the nucleotide sequence has changed more dimly than that of the bulk of the DNA molecule. This meant for that even if two organisms are only distantly related and show no comparable DNA homology, there still may be the interesting similarity in the nucleotide sequences of their rRNA cistrons. The degree of similarity that exists can, therefore, be used as a measure or of relatedness between organisms but at a level beyond that of species and at the level of genus, family, order,etc.. RNA homology experiments and RNA oligonucleotide cataloging are two modern techniques used to determine the degree of similarity between the rRNA cistrons of different organisms. These methods are complex and are being used only a few laboratories.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.