They are everywhere, in the air we breathe, in the food we eat, in the water we drink. They are on our skin and in our bodies. They make us sick and help create drugs that make us healthy. They are microorganisms.

Microorganisms or microscopic organisms are tiny living organisms that are sometimes invisible to the eye. They exist as unicellular organisms and as groups of single-celled organisms known as colonies.

Microorganisms live in almost all habitats, from the cold tundra in the polar zones, the rain forests, deserts, Savannah, and down to the deep sea.

Some are adapted to extreme temperatures and pressure as high as those found in the deep ocean. Microorganisms are very important in human life.

They aid in fermentation and decomposition of dead organic matter, they are used to produce food such as cheese from milk. They also help in the production of drugs.

Prior to the discovery of microorganisms by Antonie Van Leeuwenhoek in the 17th century, their possible existence has long since been discussed among philosophical and scientific circles.

The Jains of 5th century India postulated a theory about tiny living creatures called “nigodas” too small to be seen with the naked eyes that live on everything.

Since then several people have made great impact towards the discovery of microorganisms. Robert Hooke used microscopy to observed microbial life in moulds.

Louis Pasteur discovered in his broth experiment little spore like particles growing it the broth exposed to air compared to the broth covered with a filter; this was the basis of Germ Theory of Disease.

Robert Koch established that microorganisms cause diseases when he observed that cows with anthrax contain large numbers of the bacteria Bacillus anthracis. Martinus Beijerinck discovered virus and the development of enrichment cultures.

Types of Microorganisms

Microorganisms differ from each other in size, habitat, method of reproduction, structure, metabolism and many other characteristics.

However, there are several broad classes in which all microorganisms are grouped into. They are bacteria, virus, archaea, protozoa and algae.

1. Bacteria


These are unicellular microorganism with no cell nucleus. Microorganisms like these are described as prokaryotic. Bacteria exists in different shapes; bacillus (rod shape), coccus (spherical shape), vibrio (curved shape), spirilla (spiral shape).

Many bacteria reproduce by binary fussion (such as in Amoeba) or by spore formation. Optimum conditions for bacteria can make them grow and reproduce rapidly.

Motility in bacteria is by various means such as pseudopoda (fake feet), whip-like flagella or by cilia. Bacteria can also be classified by their response to oxygen.

Aerobic bacteria thrive and survive in the presence of oxygen while anaerobic bacteria can survive without oxygen. Facultative bacteria can survive in both conditions.

Bacteria are most known for being the causal agent for many diseases in plants and animals. However, there are beneficial bacteria such as those found in the human digestive system.

2. Virus


Virus are microorganisms that contain a strand of DNA or RNA surrounded by a protein coat. Viruses can survive in any environment but require a host organism to become active.

Many experts consider viruses non-living organisms because of this characteristic. A virion (virus particle) infects its host by incoperating itself into the host cell and co-opting its cellular mechanism.

Virus can infect humans, animals, plants and even other microorganisms. Viruses are responsible for numerous diseases and epidemic such as HIV, Ebola, the Black Plague etc.

In many cases, the immune system detects the presence the virus and takes action, leaving the infected person with symptoms such as cold, sore throat or influenza.

3. Fungi

These are eukaryotic (containing nucleus), multicellular organisms. Fungi are larger than other microorganisms and can sometimes be seen with the naked eye. Examples include mushrooms and moulds.

Many fungi are decomposers and obtain nutrients from their environment either through the symbiotic relationship with plants or the parasitic relationship with the host cell.

Fungi are useful in the food industry, for example, yeast is used in the production of beer and other consumables.

Also, there many edible species of mushrooms. However, fungi can cause problems for those with compromised immune system and contains pathogens that are dangerous to plants.

4. Protozoa


Protozoa are classified under protists. Protists are unicellular eukaryotes that are not animals, plants and fungi.

Another example of protists is algae. Protozoa have a nucleus, complex organelles, and obtain nourishment by absorption or ingestion through specialized structures.

They make up the largest group of organisms in the world in terms of biomass, numbers,and diversity. Movement in protozoa is by the beating of the hair-like cilia or by the whipping movement of the flagella.

The cell wall of protozoa is made up of cellulose. Many protozoa are harmless but some are causal agents of diseases with the most popular being Plasmodium sp. the causal agent of malaria.

5. Algae

Algae Microorganisms

Algae, also known as blue-green algae or cyanobacteria, They are unicellular or multicellular protists. Their cell walls are made up of cellulose and they obtain nutrients via photosynthesis.

Many products consumed by humans contains ingredients derived from algae such as alginic acid and carrageenan, found in some brands of ice-cream and salad dressing, Also, agar, a gel used to make microbial cultures in the laboratory is derived from algae.

Applications of Microoganisms

Microorganisms are majorly used in the food industry. They are used in a fermentation process to produce yoghurt, cheese and alcohol. They are also used in the treatment of polluted water.

Anaerobic bacteria are introduced into polluted water at water treatment plants to break down and dissolve substances. Microorganisms are also used in bioremediation.

This is the process by which microorganisms are introduced into polluted soil to break down substances. Enzymes, chemicals and other bioactive molecules are produced by microoganisms.

Microorganisms are essential tools in biotechnology, agriculture, genetics and molecular biology. Microorganisms have been used in biological warfare.

In the middle ages, dead bodies were thrown over city walls with large catapults to force the city to surrender. In recent times, certain countries have been accused of being involved in biological warfare.

Other cases of bioterrorism includes the release of anthrax by Aum Shinyirko in Tokyo, Japan in 1993 and 1984 Rajineeshee bioterror attack in Oregon, USA.