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Chapter: 11th Microbiology : Microbial Taxonomy

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Taxonomic Systems

Classical Taxonomy, Numerical Taxonomy, Molecular Taxonomy.

Taxonomic Systems

Classical Taxonomy

Classical taxonomy is a method of classification based on morphology, physiology, biochemical and ecological characteristics of the microorganisms.

Morphological Characteristics: The structural characteristics are the usual tools which help in classification. Cell morphology gives little  information about  phylogenetic relationship.  The first step in identification of bacteria is differential staining.

Physiological and metabolic characteristics: These characteristics are useful because they are directly relatedtonatureandactivityofmicrobial enzymes and transport protein. Since proteins are gene products, analysis of  these  characteristics  provides  an indirect comparison of microbial genomes.

Biochemical characteristics: Enzy-matic  activities  are  widely  used  to differentiate bacteria. Bacteria can be separated into separate species by various biochemical  tests. Example: Carbohydrate fermentation ability of bacteria.

Ecological characteristics: Many properties  are  ecological  in  nature since they alter the relation of microorganism to their environment. Microorganisms living in various parts of the human body markedly differ from  one  another  and  from  those growing in freshwater, terrestrial and marine environments.

Prokaryotes have only a few structural characteristics and these characteristics are subject to rapid change due to change in environment. In classifying prokaryotes, metabolic reactions, genetic relatedness and other specialized properties are used (Figure 8.4).


 

Numerical Taxonomy

The objective classification system deals with the grouping by numerical methods of taxonomic units based on their character and does not use subjective evaluation of their properties. To be more objective about grouping bacteria, the scientists determine many characteristics (usually 100 to 200) for each strain studied, giving equal weightage for each character. Then by using computer %similarity is calculated (%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. Those strains having a high %S to each other are placed into groups; and those groups having a high %S to each other are in turn placed into larger groups. Numerical taxonomy also yields classification that has a high degree of stability and predictability.

 

Molecular Taxonomy

Molecular techniques in the field of biology has helped to understand genetic relationship between the numbers of different taxonomic categories.

DNA and protein sequencing, immunological methods, DNA-DNA or DNA-RNA hybridization methods are very helpful in studying different species.

 

The data or information from such studies are used to construct phylogenetic tree (a branching diagram showing the evolutionary relationship among various biological species based on similarities and difference in their physical or genetic characteristics).

A classification technique that is widely used is DNA base composition which is expressed as the percentage of Guanine plus Cytosine (G+C). It is a fixed property that reveals the degree of species relatedness. Ribosomal RNA sequencing is used to determine the diversity of organisms and the phylogenetic relationship. Basically ribosomes consists of two subunits, each of which is composed of protein and a type of RNA. Specific base sequences called as signature sequences are found in all groups of organisms. These unique DNA sequences are 5-10 bases long and found in 16s rRNA location and unique to major groups of prokaryotic organisms.

Nucleic acid based detection methods help in the detection of genomic materials. The 16s rRNA gene sequencing has been established as the “gold standard” for identification & taxonomic classification of microbial species.

 

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