Systematic Classification of Microorganisms, Biology tutorial


In order to make sense of the variety of organisms, it is essential to group similar organisms altogether and organize such groups in a non-overlapping hierarchical arrangement. One of the tools required to carry out this grouping is a reliable categorization method. The Swedish botanist Carl von Linne and Carolus Linnaeus, as he is often termed, build up the first natural categorization based largely on anatomical features in the middle of the 18th century. The word systematic is frequently used for taxonomy. Though, most of the taxonomists define systemics in more general word as 'the scientific study of organisms having ultimate objective of characterizing and arranging them in an orderly way'. Every study of the nature of organisms, if the knowledge gained is employed in taxonomy, is a portion of systematics.

Therefore, systematics encompasses discipline like ecology, morphology, epidemiology, molecular biology, biochemistry, and physiology.

The Polyphase Taxonomy:

The polyphasic approach to taxonomy employs three types of methods: phenotypic, genotypic and phylogenetic for the recognition and explanation of bacteria. Phenotypic analysis observes the metabolic, morphological, physiological and chemical features of the organisms. Genotypic analyses consider comparative features of cells at the level of genome.

These two types of analyses group organisms on the base of similarities. They are complemented through phylogenetic analysis that places organisms in a framework of evolutionary relationships. The ecology and habitat of the organisms is as well employed in the polyphasic taxonomy.

Methods of Classification:

1) Phenotypic classification: This is the grouping of microorganism's altogether based on the mutual similarity of the phenotypic features. This categorization system succeeded in bringing order to biological diversity and categorized the function of morphological structures. Organisms sharing numerous features make up a single group or taxon.

2) Phylogenetic classification: It compares organisms on the base of evolutionary relationships. The word phylogeny refers to the evolutionary growth of species. This process is restricted due to lack of good fossil records.

3) Genotypic classification: It compares the genetic similarity among organism's individual genes or whole genomes can be compared. 

4) Numeric taxonomy: This is the grouping of taxonomic units into taxa on the basis of their character state by numerical processes. Information regarding the properties of organisms is transformed into a form appropriate for numerical analysis and they are compared by means of a computer. The resultant categorization is based on general similarity as judged by the comparison of numerous characteristics each given equivalent weight. The outcome of numerical taxonomic analysis is frequently summarized by a threadlike diagram termed as a dendogram.

Taxonomic Ranks:

The categorization of microbes includes placing them in hierarchical taxonomic levels. Microbes in each and every level or rank share a common set of particular features. The highest rank is the domain and all the prokaryotes fit in to either the Bacteria or the Archaea. In each domain, each microbe is assigned in descending order to a phylum, class, family, order, genus and species.

The fundamental taxonomic group in the microbial taxonomy is the species.

A prokaryotic species is a collection of strains which share numerous stable properties and distinct considerably from other groups of strains.

A strain comprises of the descendants of a single, pure microbial culture. Each and every species is assigned to a genus, the subsequent rank in the taxonomic hierarchy.

A genus is a well-stated group of one or more species which is clearly separated from the other genera.

Taxonomy groups of higher rank than genus are illustrated as:

Family: A group of similar genera

Order: A group of similar families

Class: A group of similar orders

Phylum: A group of similar classes

Domain: A group of similar phyla

Methods for finding Microbial Taxonomy and Phylogeny:

Numerous approaches are employed in categorizing and recognizing microorganisms. For clarity, these have been categorized into two groups: Classical and molecular.

1) Classical Characteristics:

Classical approaches to taxonomy make utilization of morphological, biochemical, ecological, physiological and genetic features. They are quite helpful in routine recognition and might give phylogenetic information as well.

a) Morphological Characteristics:

Morphological characteristics are significant in microbial taxonomy for lots of reasons. One, morphology is simple to study and examine, specifically in eukaryotic microorganisms and the more complex prokaryotes. Moreover, morphological comparisons are important as structural features based on the expression of numerous genes, are typically genetically stable. Therefore, morphological similarity is often a good sign of phylogenetic relatedness.

b) Physiological and Metabolic Characteristics:

Physiological and metabolic characteristics are extremely helpful as they are directly associated to the nature and activity of microbial enzymes and transport proteins. As proteins are gene products, analysis of such features gives an indirect comparison of the microbial genomes.

c) Ecological Characteristics:

The capability of micro-organisms to colonize a particular environment is of taxonomic value. Several microbes might be much similar in many other respects however inhabit various ecological niches, recommending that they might not be as closely associated as first suspected.

d) Genetic Analysis:

However prokaryotes don't reproduce sexually, the study of chromosomal gene exchange via transformation, conjugation and transduction is at times helpful in their categorization.

2) Molecular Characteristics:

Micro-organisms have left no fossil record dissimilar to evolutionary biologist studying animals and plants which have drawn from a rich fossil record to bring together a history of morphological changes. In this situation, molecular approaches serve to supplement this data; therefore molecular analysis is the only feasible modes of collecting a big and accurate data set from a number of microbes.

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