Current Status
Not Enrolled
Get Started

Microorganisms are microscopic organisms commonly known as microbes. They are present everywhere — even inside of our bodies. This diverse group of living organisms are too tiny to be seen without a microscope.

Microorganism include bacteria, viruses, fungi, algae, archaea, protists, and protozoa.

This course covers the following:

  • Characteristics of microorganisms and classify them
  • Characteristics, classification, and life cycles of
  • Difference between algae and plants
  • Most common disease-causing viruses in humans
Microorganisms (Virus, bacteria and fungi)
Figure 1. Microorganisms (Virus, bacteria and fungi)

The study of microorganisms is called microbiology.  Anton van Leeuwenhoek discovered microorganism in 1675 for the first time.

Several new discoveries show microorganisms to be crucial not only for scientific advances to understand and cure various diseases, but also for maintaining our ecosystem.

Most microorganisms are unicellular, with a few exceptions like Dictyostelium.

There are a few macroscopic unicellular organisms too like, Acetabularia, Xenophyophores, Gromia sphaerica (amoeba) etc.

Microorganisms have existed on earth since 1.2 to 1.5 billion years ago. Most microbes are harmless to plants, humans, and other animals but many are pathogenic.

Useful Bacteria and harmful Bacteria (enteric bacteria, Intestinal flora, Gut flora, probiotics) image illustration
Figure 2. Useful Bacteria and harmful Bacteria (enteric bacteria, intestinal flora, gut flora, probiotics) image illustration

Usefulness of microorganisms

Microorganisms are vital for life on Earth. They are misunderstood as being only disease-causing agents. A few microorganisms are pathogenic, but many more play important roles in various ecological processes, maintaining human health, etc.

Following is a list of some of the ways microorganisms are useful.

  1. Microorganisms are the foundation of the food chain that feeds all living things on earth.
  2. Microorganisms are important for digestion of food in human beings. Lactobacillus reuteri are found in breast milk and gut flora.
  3. The yeast Saccharomyces cerevisiae has been used for a long time for making bread and alcoholic beverages.
  4. Microorganisms are used in treatment of sewage plants. They are responsible for digestion and oxidation of organic materials.
  5. Several viruses are used as vectors in gene therapy. Microorganism (e.g. Agrobacterium tumefaciens) can be used to produce transgenic plants.
  6. The lactic acid bacteria, such as Lactobacillus and Bifidobacteria, are used in making fermented milk products, such as buttermilk, sour cream, cheese, and yogurt. There are more than 80 species of the Lactobacillus genus of probiotics.
  7. Microorganisms digest harmful chemicals, such as pollutants and chemical wastes produced by the industry through a process known as bioremediation.
  8. Leguminous plants, such as soybeans, peas, clover, alfalfa, and beans, form symbiotic relationship with nitrogen-fixing bacteria such as Rhizobium. Nitrogen compounds produced are used by the plant.
  9. Streptomyces species alone produce more than several antibiotics, including cycloheximide, cycloserine, haymycin, erythromycin, bacitracin, kanamycin, lincomycin, neomycin, streptomycin, and tetracycline etc. Besides being used in the treatment of human diseases, antibiotics can also work as food preservatives.
  10. Heat-stable enzymes can be produced from thermophiles (bacteria living in extremely hot environment).
  11. In medical field, microorganisms are used to cure diseases, such as cancer and anthrax. They are also used in synthesis of steroid hormones, e.g. Actinomycetes, Aspergillus, Streptomyces, and Gliocladium.
  12. In microbial mining, acidophilic bacteria, such as Thiobacillus ferroxidans are used to obtain copper from their ore.
  13. Organic acids (acetic acid, citric acid, lactic acid etc) and vitamins such as vitamin B and Riboflavin (B2) are produced by a number of microorganisms, such as Eremothecium ashbyii and Closteridium bytyricum).
  14. Enzymes such as lactase, lipase, protease, invertase etc are produced by microorganisms.
  15. Microorganisms play a significant role in the formation of coal and petroleum.


Bacteria are microscopic, single-celled organisms that exist almost everywhere. They are a type of prokaryotes. They lack nucleus and other organelles within the cell. Most bacteria have cell walls that contain peptidoglycan.

Bacteria are classified depending upon factors like shape, pH tolerance etc.  There are thousands of species of bacteria and they contain the genetic information in a single loop of DNA.

An extra chromosome, a plasmid, often contains genes that give the bacterium some advantage over other bacteria, like antibiotic resistance.

They have several forms of reproduction, but they mainly reproduce by binary fission.

In this process, the bacterium, which is a single cell, divides into two identical daughter cells.

Vector illustration of Salmonella Bacteria
Figure 3. Salmonella bacteria


Viruses are non-cellular, self-replicating, obligate, intracellular parasitic organisms, containing DNA or RNA and a capsule (protein coat).

They are considered as non-living organisms by several experts. They have a very simple structure and are smallest of all the microbes.

Viruses cannot reproduce outside a host cell and cannot metabolize on their own.

Depending on the symmetry, they are of three types: cubical (herpes, adenovirus), helical (Tobacco mosaic virus and Influenza virus), and complex (pox virus and bacteriophages like T2, T4, and T6).

Vector illustrations of viruses: bacteriophage, ebolavirus, hepatitis, rotavirus, adenovirus, papillomavirus and influenza
Figure 4. Viruses: bacteriophage, ebolavirus, hepatitis, rotavirus, adenovirus, papillomavirus and influenza


The term algae (Latin for seaweeds) was first introduced by Linnaeus in 1753. Algae comprise of a large heterogeneous group of plants, which are diverse in habitat, size, organization, physiology, and reproduction.

They can fall both under prokaryote and eukaryote categories.

Almost all algae are aquatic, but some (e.g. Chara globularis) are found on land. All algae contain a pigment called chlorophyll and they make their own food by photosynthesis.

Rhodophyceae, phaeophyceae and charophyceae etc are unicellular forms of algae, whereas volvox, pandorina, eudorina etc are multicellular algae.

Underwater Seaweed (Algae) in ocean
Figure 5. Underwater Seaweed (Algae) in ocean


Archaea is also known as Archaebacteria. They are the most primitive prokaryotes.

Archaea can be spherical, rod, spiral, lobed, rectangular or irregular in shape. Archaebacteria are characterized by absence of peptidoglycan in their wall, instead the cell wall contains protein and non-cellulosic polysaccharides.

They are classified into three major types – methanogens, halophiles, and thermoacidophile.

Methanogens live in marshy areas. Halophiles live in salt rich areas.

Thermoacidophile live in hot Sulphur springs as they have dual ability to tolerate high temperature as well as high acidity. In ruminants, they help in the digestion of cellulose.

Archaebacteria are used in the production of gas from dung and sewage.

 Layers of yellow ring and brown bacterial mat caused by thermophile bacteria in hot water
Figure 6. Layers of yellow ring and brown bacterial mat caused by thermophile bacteria in hot water


Protists are aquatic microscopic organisms that are single-celled, syncytial (coenocytic; a mass of cytoplasm), or multicellular. They are covered by plasma membrane and may be an outer covering of pellicle, cuticle, shell or cellulose wall.

They contain organelles such as mitochondria, golgi complex, endoplasmic reticulum, ribosomes, etc. Photosynthetic forms of protists contain chloroplasts with internal thylakoids. Cilia and flagella occur in several forms.

Protists are subdivided into several groups based on similarities to the higher kingdoms, such as Protozoa, Protophyta, and Molds.


Protozoa are single-celled organisms. They are of many shapes and sizes and live in a wide variety of moist habitats including fresh water, marine water, and soil. They exhibit all types of symmetry. Body may be naked or is covered by a pellicle or a test, made of silica or calcium carbonate. Locomotor organelles may be flagella (e.g., Euglena), cilia (e.g., Paramoecium), pseudopodium (e.g., Amoeba) or altogether absent in parasitic forms (contractile Myonemes).

Ciliates and amoeba (Protists) in their natural habitat
Figure 7. Ciliates and amoeba (Protozoa) in their natural habitat