Microorganisms are tiny organisms that are invisible to the naked eye. These organisms are incredibly diverse and can be found almost anywhere, from the soil beneath our feet to the air we breathe. Some microorganisms are harmless or even beneficial, while others can cause serious illnesses and diseases.

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Microorganisms Definition

Microorganisms, also known as microbes, are living organisms that are too small to be seen with the naked eye. They are found almost everywhere on earth, from the depths of the ocean to the soil in your backyard.

Microorganisms can be single-celled or multicellular, and they come in a wide range of shapes, sizes, and forms. They are classified into four major groups: bacteria, viruses, fungi, and protozoa.

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Types of Microorganisms

As mentioned earlier, microorganisms are classified into four major groups: bacteria, viruses, fungi, and protozoa.


Bacteria are single-celled microorganisms that can take on a wide variety of forms and can be found in a variety of environments. They are found in soil, water, and living organisms, including humans. While most bacteria are harmless or even beneficial, some can cause diseases such as tuberculosis, strep throat, and food poisoning.


Viruses are tiny particles that can only replicate inside a living host cell. They are responsible for a wide range of diseases, including the flu, measles, and HIV. While they are not technically alive, viruses are considered to be microorganisms due to their ability to cause disease.


Fungi are multicellular organisms that include molds, yeasts, and mushrooms. They are found in soil, water, and living organisms, including humans. While most fungi are harmless, some can cause diseases such as athlete’s foot and ringworm.


Protozoa are single-celled organisms that can take on a wide variety of forms and can range in size and shape. They are found in soil, water, and living organisms, including humans. While most protozoa are harmless, some can cause diseases such as malaria and amoebic dysentery.

Four major groups of microorganisms

List of Microorganisms

There are countless species of microorganisms, and new ones are discovered all the time. Some of the most well-known microorganisms include:

Bacteria: E. coli, Streptococcus, Staphylococcus, Salmonella

Viruses: Influenza virus, HIV, Hepatitis B, Herpes

Fungi: Candida, Aspergillus, Penicillium

Protozoa: Plasmodium, Entamoeba, Giardia

Microorganisms in Soil

Bacteria – such as Rhizobium, Bacillus, and Streptomyces

Fungi – such as Aspergillus, Penicillium, and Trichoderma

Protozoa – such as Amoeba and Paramecium

Algae – such as Chlamydomonas and Spirogyra

Viruses – such as bacteriophages and plant viruses

Microorganisms in Water

Bacteria – Escherichia coli, Salmonella, Vibrio cholerae

Algae – Dinoflagellates and Diatoms

Protozoa – Cryptosporidium and Giardia

Fungi – Aspergillus and Candida

Viruses – Norovirus and Hepatitis A virus

Microorganisms in Food

Bacteria – Salmonella, Escherichia coli, and Listeria monocytogenes

Fungi – Aspergillus, Penicillium, and Candida

Yeasts – Saccharomyces cerevisiae and Zygosaccharomyces bailii

Viruses – Norovirus and Hepatitis A virus

Molds – Rhizopus and Alternaria

Harmful and beneficial microorganisms

Pathogenic Microorganisms

Pathogenic microorganisms are microorganisms that cause disease. Protozoa, bacteria, viruses, and fungi are all examples for them. Pathogenic microorganisms can be transmitted through contact with contaminated surfaces, food or water, or through the air. Some examples of pathogenic microorganisms include Salmonella, E. coli, Influenza virus, HIV, and Candida.

Characteristics of pathogenic microorganisms

Pathogenic microorganisms are microorganisms that can cause disease in humans, animals, or plants. These microorganisms have several characteristics that allow them to cause disease.

  • Ability to invade host tissue: Pathogenic microorganisms can invade and colonize host tissues, allowing them to cause damage and disrupt normal bodily functions.
  • Production of toxins: Many pathogenic microorganisms produce toxins that can cause tissue damage and interfere with normal cellular function.
  • Ability to evade host immune response: Pathogenic microorganisms have evolved mechanisms to evade or suppress the host immune response, allowing them to persist and cause disease.
  • Rapid reproduction: Pathogenic microorganisms can reproduce quickly, allowing them to rapidly multiply within the host and cause widespread damage.
  • Ability to adapt to changing environments: Pathogenic microorganisms can adapt to changing environmental conditions, allowing them to survive in different host species and geographical locations.
  • Resistance to antibiotics: Some pathogenic microorganisms have developed resistance to antibiotics, making them difficult to treat and control.

Nonpathogenic microorganisms

Non-pathogenic microorganisms are microorganisms that do not cause disease in humans, animals, or plants. These microorganisms can still have important roles in the ecosystem and can provide many benefits, such as helping with nutrient cycling, aiding in digestion, and producing useful substances.

Examples of non-pathogenic microorganisms

  • Commensal bacteria – which live in and on the body without causing harm, and may even provide benefits such as aiding in digestion and preventing colonization by harmful bacteria.
  • Environmental bacteria – which live in soil, water, and other natural environments and help with nutrient cycling and decomposition.
  • Fermentation microorganisms – which are used in the production of food and beverages such as cheese, yogurt, and beer.
  • Bioremediation bacteria – which can help clean up pollutants and contaminants in the environment.
  • Some types of fungi – such as mushrooms, which can be edible and provide nutrients.
  • Algae – which can be used as a food source for humans and animals.

Beneficial Microorganisms

Not all microorganisms are harmful. In fact, many microorganisms are beneficial to humans and other organisms.

Examples of beneficial microorganisms

  • Probiotics: These are live bacteria and yeasts that are good for your digestive system.
  • Fermenting bacteria: These bacteria are used to make foods such as yogurt, cheese, and sauerkraut.
  • Nitrogen-fixing bacteria: These bacteria help plants to absorb nitrogen from the soil, which is essential for their growth.
  • Decomposers – such as certain types of bacteria and fungi, which break down organic matter and recycle nutrients back into the soil.
  • Plant growth-promoting bacteria – such as Bacillus and Pseudomonas, which can help plants absorb nutrients and resist disease.
  • Bioremediation bacteria – such as Deinococcus radiodurans and Pseudomonas putida, which can break down pollutants and contaminants in the environment.
  • Yeasts – such as Saccharomyces cerevisiae, which is used in baking and brewing, and other yeasts which can help control spoilage and improve fermentation.
  • Bacteriophages – which are viruses that can infect and kill harmful bacteria, and are being studied as a potential alternative to antibiotics.

Are Viruses Microorganisms?

While viruses are not technically alive, they are still considered to be microorganisms due to their ability to cause disease. Unlike bacteria, viruses cannot survive outside of a living host cell. They are essentially a bundle of genetic material

Questions on microorganisms

Disease causing microorganisms are called

pathogens. These are microorganisms such as bacteria, viruses, fungi, and protozoa that can cause diseases in humans, animals, and plants. Pathogens can enter the body through various means such as ingestion, inhalation, or through breaks in the skin.

Once inside the body, they can multiply and cause damage to cells, tissues, and organs, leading to a range of illnesses and diseases. It is important to take measures to prevent the spread of pathogens, such as practicing good hygiene, getting vaccinated, and avoiding contact with sick individuals.

Foods that allow microorganisms to grow are known as

Foods that allow microorganisms to grow are called perishable foods. Perishable foods are those that are capable of spoiling quickly due to the presence of microorganisms, such as bacteria, yeasts, and molds. These microorganisms can thrive in foods that are high in moisture, nutrients, and protein, and can cause spoilage, discoloration, and off-flavors.

Some examples of perishable foods include fresh fruits and vegetables, dairy products, meat, poultry, fish, and eggs. Proper storage and handling of perishable foods is essential to prevent the growth of harmful microorganisms and ensure food safety. This includes keeping perishable foods refrigerated or frozen, cooking them to the proper temperature, and consuming them within a certain timeframe.

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About Author

Anuradhika Lakmali

Anuradhika Lakmali is a co-founder of Science A Plus learning network. She is working as a government teacher and has interest in chemistry, biology, phisics and self development.