Introduction to Microbiology, Biology tutorial


Microbiology is basically the study of microorganisms. These are organisms too small to be observed clearly through the unaided eyes. Microorganisms comprise fungi, bacteria, algae, protozoa and bodies at the borderline of life which are termed as viruses. The cell is the basic unit of life. Most of the microorganisms are unicellular, in unicellular organisms all the life processes are carried out by a single cell. Though, some are multicellular, having more than one cell. This unit observes the definition of microbiology, kinds of microbial cells, the various groups of microorganisms and the domains in which they are positioned and why viruses are not positioned in any of the domains.


Microorganisms are organisms too small to be seen evidently by the unaided eyes. They are extremely small life forms so small that individual microorganisms can't be seen devoid of magnification. They comprise bacteria, fungi, algae, protozoa and viruses. Some of the microorganisms though, similar to the eukaryotic microorganisms are visible devoid of magnification. Illustrations are bread moulds and filamentous algae.

Microorganisms are Cells:

The cell is the basic unit of life; a single cell is the entity isolated from other cells. Two basic different types of cells exist among microorganisms are: prokaryotic and eukaryotic.

Prokaryotes: Such microbial cells are deficient in membrane-bound nucleus and organelles.

Eukaryotes: Have a membrane-bound nucleus and organelles.

Prokaryotic cell

Eukaryotic cell

1) Size : Usually small ( 1-10 µm) Where 1 µm = 10-6 m

1) Size: Usually large ( 5-100 µm)

2) Nuclear region: It is not well stated and known as nucleoid.

2) Nuclear region: well stated and surrounded by a nuclear membrane.

3) Chromosome: single

3) More than one chromosome

4) Membrane-bound cell organelles absent.

4) Membrane-bound cell organelles present.

Classification Systems for Microorganisms:

The Five Kingdom System of Classification:

Based on the cell type and mode of nutrition, there was an organization of the five kingdom system of categorizing organisms in which we encompass: 

1) Monera

2) Protista 

3) Fungi

4) Planta

5) Animalia

Microorganisms apart from for viruses that are acellular and encompass their own categorization system were positioned in the first three kingdoms.

Three Domains System of Classification:

Presently, all the way through advances in the cell biology, biochemistry and genetics, microorganisms are now positioned into three domains, each of which includes of various kingdoms.

The domains are: 

1) Bacteria (prokaryotic - true bacteria)

2) Archaea (prokaryotic - ancient bacteria)

3) Eucarya (eukaryotic)

Domain Bacteria:

a) They are prokaryotic. 

b) They are single celled organisms.

c) They are deficient in membrane bound nucleus and organelles.

d) Most encompass cell wall which includes peptidoglycan.

e) They are found in the soil, air and water and on other living organisms.

f) Some are injurious whereas others are beneficial to man.

Domain Archaea:

a) They were formerly recognized as Archaebacteria.

b) They are prokaryotic.

c) They are single celled organisms.

d) They are deficient in membrane bound nucleus and organelles.

e) They are deficient in peptidoglycan in their cell walls.

f) They encompass unique membrane lipids.

g) Some contain unusual metabolic features, example: methanogens that produce methane gas.

h) Most are found in extreme environments.

Domain Archaea is differentiated from bacteria based on:

a) Differences in the ribosomal RNA sequences.

b) The absence of cell wall peptidoglycan.

c) The presence of unique membrane lipids.

Domain Eucarya:

The main groups of microorganism in this domain are fungi and protists.

Protists:  Such groups of microorganisms are unicellular protozoa, algae, slime moulds and water moulds.


a) They are simple organisms.

b) Mostly unicellular.

c) They are photosynthetic altogether having Cyanobacteria.

d) They produce around 75 percent of the plant's oxygen. 

e) Generally found in the aquatic environment.

f) They are mainly producers in food chains in the aquatic habitat. 


a) They are unicellular.

b) Eukaryotic organisms and animal like.

c) They are generally motile.

d) Some are free living whereas some are pathogenic.

Slime Moulds: They are protists which contain various forms at different phases of their life cycles. At a phase they are similar to protozoa and at other phase similar to fungi.

Water Moulds:

These are found on the surface of fresh water and moist soils. They nourish on decaying vegetation like mulch and logs.


a) These are the microorganisms which range from unicellular forms such as yeasts to moulds and mushrooms that are multicellular having thread like structures termed as hyphae.

b) They absorb nutrients from the environments. 

c) Most of them play valuable roles whereas others cause diseases in animals, plants and human.


a) They are acellular entities (that is, non cellular).

b) They lack the basic structure of living cell however just carry out functions of the living organisms if in living cells.

c) They are the smallest among all the microorganisms (that is, 10,000 smaller than a typical bacterium).

d) They can merely be seen by using the electron microscope.

e) They can cause numerous diseases of animals, plants and humans.

f) Entities are not located in any of the domain however are categorized on a separate system.

They can cause most of the diseases of animals, plants and humans.

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