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Classification of Microorganisms – Taxonomy


Many characteristic features are used in classifying and identifying microorganisms. In general, these characteristic features have been divided into two major categories such as classical and molecular characteristics.

Classical characteristics

The classical type of approaches such as morphological, physiological, biochemical, ecological and genetic characteristics have been widely employed to study microbial taxonomy and it also provide phylogenetic information of microorganisms.
Morphological characteristics:- Morphological features are important in microbial taxonomy for many reasons. Morphology is easy to study and analyze both eucaryotic and procaryotic microorganisms. Many different morphological features are used in the classification and identification of microorganisms. Some of these features are cell size, cell shape, colonial morphology, ultrastructural characteristics, staining behavior, cilia and flagella, mechanism of motility, color etc.
Physiological and Metabolic characteristics:-

Physiological and metabolic characteristics are very useful because they are directly related to the nature and activity of microbial enzymes and transport proteins. Because proteins are gene products, analysis of these characteristics provides an indirect comparison of microbial genomes. Some of the physiological and metabolic characteristic features are carbon and nitrogen sources, cell structure, energy sources, fermentation product, nutritional type, growth temperature optimum and range, luminescence, motility, osmotic tolerance, oxygen requirements, pH optimum and growth range, photosynthetic pigments, salt tolerance, sensitivity to metabolic and antibiotics etc.

  Ecological characteristics:-

Microorganisms are well associated and growing in terrestrial fresh water and marine environments. The taxonomically important ecological properties are life cycle patterns, the nature of symbiotic relationship, the ability to cause decease in particular host and habitat preference such as the temperature, pH, oxygen and osmotic concentration.

Genetic characteristics:-

Most eucaryotes are able to reproduce sexually, hence genetic analysis has been of considerable usefulness in the classification of these type of microorganisms. However, procaryotic do not produce sexually and chromosomal gene exchange (through transformation and conjugation) is sometimes useful in the classification of procaryotes.

Molecular characteristics

The recent molecular approaches such as comparison of protein, nucleic acid base composition, and nucleic acid hybridization and sequencing are the most powerful molecular tools have been employed to study the taxonomy of some microbial groups, especially important for the procaryotic taxonomy.

Further, they are classified based on their risk categories,
Harmless microorganisms (EFB class 1)
Micro-organisms that have never been identified as causative agents of disease in man and that offer no threat to the environment.
Low-risk microorganisms (EFB class 2)
Micro-organisms that may cause disease in man and might, therefore, offer a hazard to laboratory workers. They are unlikely to spread in the environment. Prophylactics are available and treatment is effective.
Medium-risk microorganisms (EFB class 3)
Micro-organisms that offer a severe threat to the health of laboratory workers but a comparatively small risk to the population at large. Prophylactics are available and treatment is effective.
High-risk microorganisms (EFB class 4)
Micro-organisms that cause severe illness in man and offer a serious hazard to laboratory workers and people at large. In general effective prophylactics are not available and no effective treatment is known.
Environmental-risk microorganisms

Micro-organisms that offer a more severe threat to the environment than to man. They may be responsible for heavy economic losses. This group includes several classes, Ep 1, Ep 2, Ep 3, to accomodate plant pathogens.


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