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Microorganisms are classified as Bacteria , Rickettsiae , Actinomycetes , Fungi , Protozoa , Algae , Viruses )

Introduction Of Microbiology 

Microbiology is the study of living organisms of microscopic size which include bacteria, fungi, algae, protozoa, and viruses. This term was introduced by the French Chemist Louis Pasteur, who demonstrated that fermentation was caused by the growth of bacteria and yeasts. He is known as the Father of Microbiology. These microorganisms may be seen only by magnifying their images with a microscope.

Microorganisms are present everywhere on earth, including humans, animals, plants, soil, water, food, and the atmosphere. Microorganisms can multiply in these habitats by utilizing different nutrients. These microorganisms may be beneficial or harmful to human life.

Microbiology is one of the largest and most complex biological science as it deals with many diverse physical disciplines. The branches that come under microbiology are categorized into “pure” and “applied” sciences.

Branches of pure sciences under microbiology are classified as bacteriology (study of bacteria), mycology (study of fungi), protozoology (study of protozoa) phycology or algology (study of algae), parasitology (study of parasitism and parasites), microbial taxonomy (study of the classification of microorganisms), genetics (study of heredity and variations) and immunology (study of the mechanism involved in the development of resistance by the body to infectious diseases). Microorganisms are classified as bacteria, rickettsia, actinomycetes, fungi, algae, protozoa, and viruses.

Bacteria :

These are unicellular, free-living small microorganisms that are visible under a light microscope. They occur in water, food, soil, air, and all-natural environments.
They grow on artificial laboratory media and multiply by binary fission. Cocci are about 1 μm in diameter and bacilli are 1 to 8 um in length and 0.1 to 0.5 μm in width.
Many bacteria cause diseases in plants, animals, and human beings. Some species play an imp in the natural cycling of elements (soil fertility) and some others may be useful in I preparation of different compounds.


Rickettsiae were first discovered in 1909 by Howard Taylor Ricketts.

They are small, Gram-negative bacilli and have properties of both bacteria and viruses. Rickettsiae are transmitted to humans primarily by arthropods such as ticks and lice and are cultivated only in living tissues (like viruses).

They are obligate intracellular parasites and multiply by binary fission. Epidemic typhus, scrub typhus, rocky mountain spotted fever, rickettsialpox, Q-fever, and trench fever are examples of diseases caused by rickettsiae.


Actinomycetes are Gram-positive bacteria with a fungal morphology.

They are nonmotile, non-capsulated, and filamentous and break into bacillary and coccoid elements forming aerial mycelium and substrate mycelium. They play a major role in the cycling of organic matter in the soil ecosystem.

They are a rich source of antibiotics, enzymes, biosurfactants, enzyme inhibitors, immune modifiers, and plant growth stimulants and help in the biodegradation of agricultural and urban wastes.

Actinomycetes are mainly found in soil, water, compost, and other extreme environments.


Fungi are eukaryotic microorganisms that exist as saprophytes, parasites, or commensals. Fungi may be simple oval cells (yeasts) or long tubular septate hyphae (molds) showing true lateral branching. They are aerobic or facultatively anaerobic.

They are cultivated in the laboratory like bacteria and reproduce by asexual and sexual processes. Fungi are essential sources of antibiotics, enzymes, acids, foods, bakery, and alcohol fermentations. Certain fungi also cause human diseases, such as athlete’s foot and thrush,


Protozoa are unicellular eukaryotic microorganisms. They are differentiated based on morphological, nutritional, and physiological characteristics. They are generally 2 to 200 µm in size and have complex life cycles.

Some protozoal species are easily cultivated in laboratory media like bacteria and others are intracellular parasites.

All protozoa require large amounts of moisture for growth and activity. They reproduce by asexual and sexual processes. Some species can cause diseases such as malaria and sleeping sickness.


Algae are unicellular or multicellular aerobic eukaryotic microorganisms that mainly occur in aquatic environments. They contain chlorophyll and are photosynthetic. Reproduction occurs through asexual and sexual processes.

Some algae can be easily cultured in the laboratory by using artificial media.

They are mainly used as food supplements and in pharmaceutical preparations. Agar, used in culture media is prepared from different types of algae.


Viruses are non-cellular small microorganisms (20 to 300 nm) that can only be seen under an electron microscope. They are 10 to 100 times smaller than most bacteria. They are obligate intracellular parasites.

Viruses comprise an ultramicroscopic fragment of nucleic acid, either DNA or RNA, surrounded by a sheath of protein.

They are responsible for many human diseases including AIDs, hepatitis, influenza, polio, and chickenpox.