Initially, the living organisms were classified as plants and animals by their visual differences in form and constitution. These were accepted as two broad kingdoms of living creatures.

This classification was made before the discovery of microorganisms, although Leeuwenhoek saw these organisms in the year 1674.

During the course of time, it became evident that the old and common division’s old the living world is insufficient because the microorganisms show similarities with plants being green and autotrophic, in one hand, and non-green and heterotrophic animals, at the other. Bacteria are included in the plant world.

Slime molds were given the middle path between plants and animals because they were considered as animals by zoologists and plants by botanists. A new third kingdom, hence, was created to group those organisms, which were neither plants nor animals.

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The new name of this kingdom was proposed as Protist. This kingdom includes all those microorganisms, which do not possess extensive structures.

The kingdom Protist contains those organisms that are differentiated from plants and animals by their lack of morphological specialization, most of them being unicellular. The Protests can be further subdivided into two clearly differentiated groups on the basis of their Cellular structure.

The higher protests resemble plants and animals and they are the eukaryotes. This group includes algae, fungi, and protozoa. The lower protests include bacteria and cyan bacteria (blue green algae), they are prokaryotes and their cellular structure is different from all other organisms.

In 1969, Whittaker proposed a 5 kingdom concept consisting of kingdom of plants, animals, fungi, protests and monera, for all organisms, on the basis of cell types: prokaryotic or eukaryotic, level of organisation- solitary and colonial, unicellular organisation or multicultural systems, nutrition type. Recently, evolutionary relationships of living organisms have been defined on the basis of ribosomal RNA sequences and other data. The characteristics o the 5 kingdoms are:

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1. Kingdom Monera or prokaryotes

Prokaryotic cells. Nutrition-absorptive, chemosynthetic, photoheterotrphic or photoautotrophic with anaerobic, facultative, microaerophilic or arobic metabolism Reproduction-asexual, genetic recombination sometimes occurs, nonmotile or motile, solitary, unicellular, filamentous, colonial or mycelial

2. Kingdom-Protista

Eukaryotic cells with solitary and colonial unicellular organisation usually aerobic. Nutrition infective, absorptive or photoautotrophic in photosynthetic plastids all forms reproduce asexually, many have true sexual reproduction with plasmogamy, karyogamy and meiosis nonmotile or motile by cilia and flagella or other means like pseudopodia. Lack embryos and complex cell functions

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3. Kingdom Fungi

Primarily multinucleated organisms with eukaryotic nuclei dispersed in a walled and septet mycelia system. Usually aerobic nutrition heterotrophic and absorptive Flagella occurring rarely or lacking Mycelia, haploid or dikaryotic Meiosis in zygote, haploid spores produced. No pinocytosis or phagocytosis.

4. Kingdom Animalia

Multicellular animals with cell wall lacking eukaryotic cells nutrition-Primarily ingestive with digestion in an internal cavity, but some forms are absorptive and some lack digestive cavity, phagocytosis and pinocytosis occurs, Reproduction by sexual method with meiosis forming gametes, haploid stages other than gametes lacking above the lowest phyla, motility based on the contractile fibrils. Extensive cellular and tissue differentiation with complex cell junctions.

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5. Kingdom-Plantae

Primarily autotrophic, multicellular with cell walls and frequently eukaryotic cells and photosynthetic plastids. Aerobic, simple, multicellular to advanced tissue differentiation, development from solid embryos reproduction primarily sexual with haploid and diploid stages, haploid stages reduced in higher members, usually nonmotile.

Woese et al. (1990) noted that the bacteria are distant from plants and animals and by contrast plants and animals are not so far from each other. This prompted them to establish new superior concept of domains over the kingdoms and proposed three domains, Archaea, Bacteria and Eukarya in 1991.

In the modern pattern of classification bacteria, cyanobacteria, actinomycetes etc. are distrubuted in the domain of bacteria; methanogens, extremely thermophilic organisms, extremely halophilic organisms etc. in the domain of Archaea or Archaeobacteria; molds, yeasts, basidiomycetes, algae and protozoa etc. in the domain of Eukarya.

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The 5-kingdom system is not accepted by all microbiologists. The major problem is its lack of distinction between Archaeobacteria and Eubacteria. This led to 6 kingdom option-Archaeobacteria (Archaea), Eubacteria, Protista, fungi, animalia and Plantae.

Archaea (archaeobacteria)

All organisms of this group are assumed to be descended from the common ancestor, the so called progenotes. The organisms are primitive, cellular but possess a rudimentary, not well developed translation aparatus.

The membrane lipids are glycerol isopronyl ethers due to unusual chemical composition. The cell envelops show resistance against cell wall antibiotics and lyric agents. All lack murein in their cell walls.

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All Gram negative types composed of single layered or more complex crystalline proteins or glycoprotein subunits while all Gram positive ones have pseudomurein, methanochondrin or heterpolysaccharides. The size of the genome of archaea varies from 0.8 X 109 to 2.4 X 109 dalton and is generally smaller than Escherichia coli. Certain methanogenic bacteria, extremely halophilic bacteria, thermophilic sulphur metabolizing bacteria etc. are categorised under this group.

Selected Methanogenic Bacteria. Archaea (a) Methanospirillum hungatei (b) Methanobrevibacter smithii; (c) Methanosarcina barkeri\ (cl) Methcinosarcina mazei\ (e) Methanobacterium bryantii; (f) Methanogenium marisnigri.

Examples of Archaea type of Halobacteria. (a) Halobacterium salinarium. A young culture that has formed long rods (b) Halococcus morrhuae.

The unique properties of living conditions of archaea are of extreme pH, temperature, salinity and pressure. This can be utilized in establishing new areas of biotechnology in production of biogas, thermophilic enzymes, leaching and biosensors.

Eubacteria

The eubacteria are true bacteria, which comprise vast majority of its kind among other properties, eubacteria have cell wall peptidoglycan containing muramic acid and membrane lipids that resemble eukaryotic membrane lipids. These organisms have been classified in various ways.

Attention, in recent times, has been given to sequences of 5S and 16S RRNA isolated from 50S and 30S subunits respectively, of bacterial ribosomes for proper classification of the eubacteria. The rRNA is almost ideal for studies of microbial evolution relatedness.

Their functional role is the same in all ribosomes. Again, their structure changes very slowly with time. Complete rRNA can be sequenced using the enzyme reverse transcriptase and DNA sequencing techniques.

Protists

The primitive eukaryotes called ‘Protists’ evolved at least l billion years ago from the prokaryotes. The step from prokaryotes to the first eukaryotes was one of the big evolutionary transitions.

It is postulated that nearly 2.5 billion years back, oxygen began slowly to accumulate in the atmosphere as a result of photosynthetic activity of the prokaryotic organisms like cyanobactria. The Protists adapted itself to the use of oxygen evolved and got transformed into the primitive eukaryotes.

Protists are microscopic organisms. They may be coloured or colorless. Their body shape also varies. Organisms are unicellular or may form colonies.

The eukaryotic cells are surrounded by plasma lemma. There may be an outer covering of pellicle, cuticle, and shell or cellulose wall. Cytoplasm shows streaming movements. It contains organelles like mitochondria, Golgi complex, endoplasmic reticulum, 80S ribosome’s etc. Controlees occur in number of types.

Photosynthetic forms contain chloroplasts with internal thylakoids. Cilia and flagella occur in a number of forms. They have typical 9+2 fibrils. Nucleus has typical organisation- nuclear I membrane with pores, chromatin, nucleolus and nucleoplasm.

The.protists include the protistan algae like dinoflagellates, diatoms and euglena like flagellates, slime molds, which may be acellular or cellular; protozoan comprising zooflagetes, sarcodines, sporozoans and ciliates

Typical photosynthetic Protist such as Euglena is schematically cut to show internal structure as revealed by the electron microscope. B: Cross section of the flagellum shows that possesses 9 double peripheral and 2 single central fibrils.

Fungi

Fungi are the eukaryotes, which are important to humans both for their beneficial or harmful effects. They are achlorophyllous, heterotrophic, spore forming organisms. Their cell walls are typically constituted of cellulose or chitin or both.

These are generally filamentous, branched, and coenocytic and the mode of reproduction is by sexual but more often by asexual means. The asexual reproductive units are motile or nonmotile spores. Fungi are heterotrophic with absorptive mode of nutrition. The reserve food materials are stored in the form of glycogen.

This group incudes diverse organism, which range in structure from unicellular yeasts to highly complex mushrooms and bracket fungi.

They degrade complex organic materials in the environment to simple organic compounds and inorganic molecules. In this way, carbon, nitrogen, phosphorus and other critical components of the dead are released and later, made available to the living world fungal hyphen with detailed structure as seen under electron microscope.

Fungi are the major cause of plant diseases. Over 5000 species of this group attack economically important crops and garden plants. Fungi also cause certain animal and human diseases.

Fungi, particularly, yeasts are essential to many industrial processes involving fermentation. It includes making of bread, (bakery) wine and beer (brewery).

Fungi also play a major role in the preparation of cheeses, soy sauce; in the commercial manufacture of organic acids (citric, Gallic etc.), certain drugs (ergometrine, cortisone) and manufacture many antibiotics (penicillin, griseofulvin etc.) and immunosuppressive drugs like cyclosporine. Besides this group of microbes has become a prolific source research since can be grown in the laboratory, when provided suitable conditions.

Cyanobacteria

Cyanobacteria or blue green algae are prokaryotic organisms containing chlorophylla, phycobilins, glycogen as storage products. Their cell walls consist of amino sugars and amino acids.

They occur in fresh water and marine habitats. Fresh water Cyanobacteria ocur as phytoplankton or as attached algae in standing or running water. In marine conditions, they are found in intertidal zones.

Morphologically, the members of Cyanobacteria are unicellular, free living or enclosed in a mucilaginous envelope. Some form a row of cells called trichomes which often get surrounded by sheath called filamens. Filaments may be branched orunbranched.

Cell walls of Cyanobacteria are mainly of peptidoglycan. In the central protoplasm are circular fibrils of DNA, without association of basic proteins (histones). The peripheral region is occupied by thylakoids. Ribosomes are 70S type and dispersed in the protoplasm.

The protoplasm contains different types of granules which vary in sizes. They are- Cyanophycin granules, carboxysomes or polyhedral bodies and phosphate bodies. They have specific functions. Besides, gas vacuoles are present in the cells of Cyanobacteria which help in buoyancy of the organisms

Cyanobacteria can produce resting spores called akinetes. They have characteristic large Vegetative cells called heterocyst. They can only reproduce asexually by hormogonia or endospors or by the fragmentation of colonies.

The cyanobacteria have great biotchnological significance since they are good nitrogen fixers. Biofertilizers are manufactured from these organisms in industrial scale.