O Level Notes : Biology - Microorganisms And Their Application In Biotechnology

There are three types of microorganisms : viruses, bacteria and  certain fungi.


There are three types of microorganisms : viruses, bacteria and  certain fungi.


Viruses cause disease in living cell. And are the smallest disease causing particles known. They are not visible under the ordinary light microscope. Viruses infect all types of organisms, from animals and plants to bacteria and archaea. although there are millions of different types. Viruses are found in almost every ecosystem on Earth and these minute structures are the most abundant type of biological entity. The study of viruses is known as virology, a sub-specialty of microbiology.


Viruses are not considered as cells as they do nit have protoplasm. A virus is made up of one nucleic acid (either DNA or RNA) surrounded by a protein coat. They may be spherical in shape (influenza virus), rod shaped (tobacco mosaic virus) or tadpole shaped (bacteriophages). The living cell, which is attacked by the virus, is called a host cell.

Outside the host cell, the virus does not carry out any characteristic of living things. However, inside the host cell, the virus can reproduce or replicate. They do this by taking over the biochemical systems of the host cells and use them to make copies of themselves. The host cell then bursts, releasing the viruses to infect the new cells.


Viral diseases include the common cold, influenza, chickenpox, dengue fever, herpes and AIDS. Antibiotics, used to destroy bacteria, are not effective against viruses. Viruses can only be destroyed by antibiotics produced by our white blood cells.


Bacteria are living cells, barely visible under the light microscope. They are larger than viruses.


A bacterium, like any other living cell has a cell surface membrane and cytoplasm. Surrounding the cell membrane is a cell wall. Some bacteria are enclosed in a slime capsule. A bacteria does not have a nuclear envelope so it does not have a true nucleus. But, it has a single, circular loop of DNA. This is the bacterial chromosome. It contains the genes that give the bacterium its special characteristics and properties. There are also smaller rings of DNA, known as plasmids, in the cytoplasm. Plasmids are of great interest to genetic engineers, because like viruses, they can be used as vectors to transfer genes between cells of two different species.

Most bacteria are non-motile. They cannot move by themselves. However, some of them are motile. They possess long, hair-like structures known as flagella. The beating of these flagella enables the bacterium to move in a fluid medium.

According to their shapes, bacteria can be classified into three general types:

  • Cocci: singular-shaped bacteria
  • Bacilli: rod-shaped bacteria
  • Spirilla: spiral shaped bacteria.


Bacteria occur in almost every environment. They are found in foods, soil, air, water and in bodies of animals (especially in the intestines). Bacteria may be:

  • Saprophytic: feeding on decaying organic matter.
  • Parasitic: causing diseases in animals and plants.
  • Autotrophic: able to manufacture their own food using energy from sun or inorganic compounds.


Some bacteria are aerobic, requiring oxygen for respiration. Whereas, some are anaerobic and there even are some that can survive in both aerobic and anaerobic conditions,

Bacteria are known for their ability to produce rapidly. They reproduce by a process called binary fission, in which the bacterial cell copies the DNA it contains and then splits in half to form two new cells. In adverse conditions, some bacteria spores, each protected by a thick wall. Such spores are very resistant to cold or heat, and very drastic conditions are necessary to kill them. If the spores settle on a suitable medium, their walls break and they begin to grow and multiply.


Fungi live as saprophytes feeding on decaying organic matter or as parasites of plants and animals. They may be unicellular e.g. yeasts, or multicellular e.g. bread mould, mushrooms and penicillium.


Yeasts are single-celled but most fungal species are multicellular.

Multicellular fungi are composed of filaments called hyphae (singular: hypha).

Hyphae may contain internal crosswalls, called septa, that divide the hyphae into separate cells. Coenocytic hyphae lack septa. The septa of many species have pores, allowing cytoplasm to flow freely from one cell to the next. Cytoplasmic movement within the hypha provides a means to transport of materials. 

The hyphae may be branched. A dense mass of hyphae is called a mycelium.

Fungi have cell walls (like plants) but the cell walls are composed of chitin, which is what arthropod (insects, crayfish, etc.) exoskeletons are composed of. The cell walls of plants and some protists are composed of cellulose.


The hyphae of some symbiotic fungi become specialized for penetrating the cells of the host. These hyphae are called haustoria.


Most fungi do not have flagella in any phase of their life cycle. They move toward food by growing toward it.



A unicellular fungi, like yeast, may reproduce by budding. Fungi may reproduce by means of spores. The spores are minute and resistant to adverse environmental conditions. They are dispersed by wind and when they settle on a suitable medium, they will germinate into new mycelia.

The fine branching hyphae spread over the surface of the substance on which they grow. The bread mould growing on a slice of bread is a common sight. The hyphae secretes enzymes to digest the organic food substances outside their bodies. This is an example of extracellular digestion. The enzyme amylase digests starch to maltose. Maltase digests maltose to glucose. Proteases digest proteins to amino acids. These simplifies food substances are absorbed by diffusion into the hyphae. Such activities of fungi enable them to play a role as decomposers.

Parasitic enzymes live on the living tissues of their hosts, e.g. the toadstool fungi living on the tree trunks and the fungus causes potato blight. Animal fungal diseases include white spots on the bodies of fish and ringworm on the bodies of man. Ringworm of the foot is also called athlete’s fppt.


Decomposers are saprophytes. They include fungi and certain bacteria, especially those that live in the soil. Although moulds and mushrooms can be seen, most decomoposers are microorganisms.


Mould and bacteria and higher level organisms like beetles, centipedes and, of course, earth worms are all busy recyclers. Locked in the tissues of every plant and animal is a wealth of nutrients, including carbon, nitrogen, and phosphorous. Living organisms require huge amounts of these and other elements in order to grow and survive.

Without the help of worms and other decomposers, every plant and animal that died would stay right where it fell. Trees, leaves, fruit, nuts, dead animals and food would just keep piling up on the ground. Not only would this create an unpleasant environment to live in, but the ecosystem would run out of the nutrients needed to keep the living organisms alive.

 Decomposers, including fungi, bacteria, and invertebrates like

earthworms, work to take apart the cells and structures that made up the dead organism. In the process of breaking down dead plant and animal tissue, decomposers not only gain energy to fuel their own life processes, but release nutrients back to the environment, where they can be used again by other organisms. All of that dead material becomes rich soil for new seedlings to grow. And the cycle starts all over again!


Sewage is normally decomposed by saprophytic microorganisms. However, the natural cycle takes a long time. In cities, sewage is drained into large settling tanks. Bacteria present in the sewage secrete enzymes to digest the solid organic matter into soluble, harmless substances and the gas, carbon dioxide. Bacteria also feed on such digested products. The liquid that results from this bacterial digestion is filtered and pathogens removed before it is discharged into the rivers or seas. The digested solid, called sludge, is removed from tanks and is used as fertilizers.






Yoghurt is made by fermenting milk, using certain bacteria like LACTOBACILLUS BULGARICUS and STREPTOCOCCUS THERMOPHILUS. Milk is incubated with these bacteria the streptococcus removes oxygen, therefore creating an anaerobic condition. Lactobacillus respires anaerobically and converts lactose into lactic acid. This acid causes milk protein, casein, to curdle producing yoghurt.


Both bacteria and fungi are used in this process. Lactobacillus is used to ferment milk sugar to lactic acid. The latter curdles milk protein. The curdled protein, together with the fats in the milk, is removed and acted upon by a mixture of bacteria and fungi to produce cheese. By varying conditions, such as temperature and using different mixtures of bacteria and fungi, different types of cheese can be produced.


The flour used in bread making contains starch, protein and amylase. The flour is mixed with water to form a dough. Yeast is then mixed with this dough. Amylase digests the starch to sugar. Lack of oxygen inside the dough causes the yeast  to respire anaerobically, fermenting the sugar to alcohol and carbon dioxide. The carbon dioxide produced causes the dough to rise, thus making cavities appear in the bread. The alcohol produced evaporates during the process.


Yeast is used un brewing. Here, yeast is mixed with sugar found in food substances used in brewing such as barley, grapes etc. In the absence of oxygen, yeast respires anaerobically to produce alcohol and carbon dioxide from the sugar. Examples of alcohols brewed in this way include beer, wine, sake and whiskey. The alcohol is separated from the fermented liquors by distillation.