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Microorganism – Definition, Types, Importance, Examples

What is Microorganism?

  • Microorganisms, often referred to as microbes, constitute a diverse and pervasive group of life forms characterized by their microscopic dimensions, typically being invisible to the unaided human eye. This class of organisms includes single-celled entities as well as multicellular colonies, and their existence, although not empirically confirmed until the advent of the microscope, has been contemplated since antiquity. Microbiology, the scientific discipline devoted to the study of microorganisms, commenced in the 1670s with Anton van Leeuwenhoek’s pioneering observations using microscopes, thereby opening new vistas for scientific inquiry and understanding.
  • One of the seminal revelations in microbiology was the substantiation, by Louis Pasteur in the 1850s, that microorganisms were responsible for food spoilage, effectively refuting the once-held hypothesis of spontaneous generation. Subsequently, in the 1880s, Robert Koch made momentous strides by demonstrating that microorganisms were the causative agents of several debilitating diseases, including tuberculosis, cholera, diphtheria, and anthrax. These groundbreaking discoveries underpin the foundational knowledge about microorganisms and their profound impact on various aspects of life on Earth.
  • Microorganisms span the phylogenetic spectrum, encompassing representatives from all three domains of life: Archaea, Bacteria, and Eukaryota. Of note, Archaea and Bacteria are exclusively populated by microorganisms. Meanwhile, within the Eukaryota domain, an array of unicellular protists and protozoans qualify as microorganisms, in addition to certain microscopic multicellular entities such as micro-animals, specific fungi, and algae. However, it is imperative to clarify that not all minute organisms are classified as microorganisms within the scientific parlance.
  • The ecological habitat of microorganisms is exceedingly diverse, ranging from polar regions to equatorial climates, arid deserts, hydrothermal geysers, subterranean rocks, and the abyssal depths of the oceans. They have demonstrated remarkable adaptations to extreme conditions such as extreme temperatures, high pressure environments, and even intense radiation, exemplified by Deinococcus radiodurans. Furthermore, microorganisms constitute the microbiota residing within and on all multicellular organisms. Notably, the discovery of microorganisms in 3.45-billion-year-old Australian rocks stands as pivotal evidence, marking the earliest known indication of life on our planet.
  • Microbes are integral to human culture and possess multifaceted roles in both public health and biotechnology. They contribute significantly to processes such as food fermentation, sewage treatment, and the production of essential bioactive compounds and enzymes. In the realm of biology, microorganisms serve as indispensable model organisms for scientific research. Regrettably, they have also been harnessed for nefarious purposes in the context of biological warfare and bioterrorism. Additionally, microorganisms play a pivotal role in soil fertility, while their presence within the human body constitutes the human microbiota, including the vital gut flora. Alarmingly, a substantial portion of infectious diseases afflicting humanity finds their origins in pathogenic microorganisms, thereby underscoring the paramount importance of hygiene measures in the context of public health.
  • In summation, a microorganism is a minuscule living entity invisible to the unaided eye. This broad classification encompasses a plethora of organisms, including bacteria, archaea, algae, protozoa, and even microscopic animals. Their pervasive presence and profound impact on diverse facets of life on Earth make microorganisms an indispensable subject of scientific investigation and scrutiny.
  • Their ability to thrive in diverse environments, from extreme conditions to the human body, underscores their adaptability and ecological significance. In light of their pivotal roles in agriculture, industry, and public health, the study of microorganisms remains an ever-evolving frontier of scientific exploration.

Definition of Microorganism

A microorganism, or microbe, is a tiny living organism that is invisible to the naked eye and can exist as single-celled entities or colonies of cells.

Discovery of Microorganisms

Microorganisms, despite their minuscule size, have had a profound impact on our understanding of life and disease. The journey of their discovery spans centuries, with contributions from various scholars and scientists.

1. Ancient Speculations: The concept of invisible life forms dates back to ancient times. By the 6th century BC, Jain scholars in India, including Mahavira, hypothesized the existence of minute organisms termed ‘nigodas’. These entities were believed to be omnipresent, even within human bodies, and had fleeting lifespans. Roman scholar Marcus Terentius Varro, in the first century BC, hinted at the presence of ‘animalia minuta’ in his work “On Agriculture”, suggesting these unseen entities might be disease carriers. Avicenna, in “The Canon of Medicine” (1020), proposed the contagious nature of certain diseases.

2. Early Modern Discoveries: The 16th and 17th centuries witnessed significant strides in the understanding of microorganisms. Turkish scientist Akshamsaddin suggested that diseases spread through tiny, unseen seeds. Girolamo Fracastoro, in 1546, theorized that diseases could be transmitted by entities that could move directly or indirectly between individuals.

However, it was Antonie van Leeuwenhoek in 1673 who revolutionized the field by being the first to observe and scientifically study microorganisms using single-lensed microscopes he crafted. His contemporary, Robert Hooke, observed and illustrated mold structures, introducing the term ‘cell’ in his book “Micrographia” (1665).

3. 19th Century: Refinement and Expansion: The 19th century was pivotal for microbiology. Louis Pasteur’s experiments debunked the spontaneous generation theory, demonstrating that microorganisms in broths came from external sources. His work laid the foundation for the germ theory of disease.

Robert Koch furthered this understanding by establishing the link between microorganisms and diseases. His meticulous experiments with anthrax led to the formulation of Koch’s postulates, criteria to determine the causative agents of diseases.

The microbial world’s vast diversity began to unravel with the work of scientists like Martinus Beijerinck and Sergei Winogradsky. Beijerinck’s pioneering work on viruses and enrichment culture techniques expanded the horizons of microbiology. Winogradsky introduced the concept of chemolithotrophy, emphasizing microorganisms’ role in geochemical processes.

4. Classification and Beyond: The discovery of unique microorganisms that defied traditional classification led to the proposal of new kingdoms. John Hogg introduced the term ‘Protoctista’ in 1860, and Ernst Haeckel later named it ‘Protista’ in 1866.

The combined efforts of these scientists, among others, have shaped our understanding of the microbial world. Their discoveries not only unraveled the mysteries of these minute entities but also laid the foundation for advancements in medicine, agriculture, and various other fields.

Types of Microorganisms

Microorganisms, often invisible to the naked eye, play a pivotal role in the ecological balance of our planet. They exist in diverse forms, each with unique characteristics and functions. This article delves into the various types of microorganisms, elucidating their nature and significance.

  1. Bacteria (Eubacteria): Bacteria, commonly referred to as eubacteria or true bacteria, are ubiquitous microorganisms. While some bacterial strains are pathogenic, causing diseases ranging from skin infections to pneumonia, many are beneficial. For instance, certain bacteria residing in the human gut aid in nutrient absorption and provide a defense against harmful pathogens.
  2. Archaea (Archaebacteria): Distinct from eubacteria, archaea were once mistakenly grouped with bacteria. Modern research indicates that archaea might share a closer evolutionary relationship with eukaryotes than with bacteria. These microorganisms thrive in diverse habitats, from human intestines to extreme environments like hot springs and saline lakes.
    • Microscopic Animals: Several animal species exist in minuscule forms, encompassing:
    • Arthropods: Examples include dust mites and microscopic crustaceans.
    • Rotifers: A subset of zooplankton.
    • Loricifera: Tiny marine animals residing in oceanic sediments.
    • Nematodes: Worm-like organisms found in various habitats.
  3. Protozoa: Protozoa, single-celled eukaryotic entities, exhibit cellular structures akin to animals and plants. Some protozoan species are pathogenic, leading to diseases such as malaria and toxoplasmosis.
  4. Fungi: Microscopic fungi, like yeast, hold significance in human culture and cuisine. Yeast, for instance, metabolizes sugars to produce carbon dioxide and ethyl alcohol, essential for baking and brewing.
  5. Molds: While molds share certain fungal attributes, they are distinct entities. Some molds are pathogenic, causing plant diseases, while others, like slime molds, demonstrate remarkable cellular cooperation, prompting scientific investigations into their problem-solving abilities.
  6. Algae: Previously classified as plants, microscopic algae are now considered separate entities. These photosynthetic organisms, possibly predating land plants, have historically contributed to atmospheric oxygen levels. While some algae purify water and produce oxygen, others can release harmful toxins.
  7. Miscellaneous Microorganisms: Historically, the term “protists” was employed to categorize microorganisms that didn’t fit into established kingdoms. However, with advancements in genetic analysis, many of these organisms were found to have closer affiliations with recognized kingdoms than with each other.

In conclusion, microorganisms, in their myriad forms, are integral to Earth’s ecosystems. Their roles, from nutrient cycling to disease causation, underscore the importance of understanding and appreciating their presence and contributions.

Harmful Microorganisms

Microorganisms, while often beneficial, can also be detrimental to human health, agriculture, and the environment. These harmful microorganisms can cause diseases, spoil food, and lead to various other problems. Here’s an overview of some of the harmful microorganisms:

1. Pathogenic Bacteria: Bacteria that cause diseases in humans, animals, and plants. Examples include:

  • Escherichia coli (E. coli): Certain strains can cause food poisoning.
  • Mycobacterium tuberculosis: The causative agent of tuberculosis.
  • Staphylococcus aureus: Can lead to skin infections, pneumonia, and food poisoning.
  • Salmonella: Often found in contaminated food and can cause salmonellosis.

2. Viruses: These are non-cellular entities that can infect all types of life forms. Some notorious viruses include:

  • Human Immunodeficiency Virus (HIV): Causes AIDS.
  • Influenza virus: Responsible for the flu.
  • Coronaviruses: Include strains causing common cold, SARS, MERS, and COVID-19.
  • Rabies virus: Causes rabies, primarily transmitted through animal bites.

3. Pathogenic Fungi: Fungi can cause infections in humans, especially in immunocompromised individuals. Examples are:

  • Candida albicans: Causes candidiasis or thrush.
  • Aspergillus: Can lead to lung infections.
  • Trichophyton: Causes athlete’s foot and ringworm.

4. Protozoa: Single-celled eukaryotic organisms that can cause diseases include:

  • Plasmodium spp.: Causes malaria.
  • Entamoeba histolytica: Leads to amoebic dysentery.
  • Trypanosoma brucei: Causes African sleeping sickness.

5. Helminths: These are parasitic worms that can reside in the intestines, blood, or tissues. Examples include:

6. Harmful Algae: Certain algae produce toxins that can be harmful to marine life and humans. For instance:

  • Dinoflagellates: Some species produce toxins leading to red tides and shellfish poisoning.

7. Harmful Microorganisms in Agriculture: Microorganisms can also harm plants and reduce crop yield. Examples include:

  • Pseudomonas syringae: Causes bacterial speck in tomatoes.
  • Fusarium spp.: Leads to wilts and rots in various plants.
  • Rice blast fungus (Magnaporthe oryzae): Affects rice crops.

8. Biodegrading Microorganisms: While they play a crucial role in breaking down organic matter, some can degrade materials like paints, polymers, and metals, leading to material damage and economic losses.

In conclusion, while microorganisms have numerous beneficial roles, their harmful counterparts underscore the importance of hygiene, vaccination, and other preventive measures to mitigate their adverse effects.

Benificial Microorganisms

Microorganisms, though often associated with diseases, play numerous beneficial roles that are essential for the sustenance of life, enhancement of health, and various industrial applications. Here’s a comprehensive look at some of the beneficial microorganisms:

1. Probiotics: These are live bacteria and yeasts that confer health benefits when consumed in adequate amounts.

  • Lactobacillus and Bifidobacterium: Commonly found in yogurts and dietary supplements, they aid in digestion and support gut health.

2. Food Production: Microorganisms are pivotal in the fermentation process of many foods and beverages.

  • Saccharomyces cerevisiae: Used in bread-making for leavening and in brewing to produce beer.
  • Lactococcus lactis: Used in cheese production.

3. Agriculture: Microorganisms enhance soil fertility and protect plants from pests.

  • Rhizobium: Forms nodules on legume roots and fixes atmospheric nitrogen, enriching the soil.
  • Mycorrhizal fungi: Forms symbiotic relationships with plants, aiding in nutrient uptake.

4. Bioremediation: Microorganisms are employed to clean up pollutants from the environment.

  • Pseudomonas putida: Used in bioremediation processes to degrade organic solvents and aromatic compounds.

5. Medicine Production: Certain microorganisms are harnessed to produce antibiotics and other medicines.

  • Penicillium chrysogenum: Produces the antibiotic penicillin.
  • Saccharopolyspora erythraea: Produces the antibiotic erythromycin.

6. Industrial Applications: Microorganisms are utilized for the production of chemicals, enzymes, and biofuels.

  • Aspergillus niger: Produces citric acid, a widely used food additive.
  • Clostridium acetobutylicum: Used in the production of biobutanol, a biofuel.

7. Waste Treatment: Microorganisms play a crucial role in wastewater treatment plants by breaking down organic matter.

  • Nitrosomonas and Nitrobacter: Convert ammonia to nitrite and nitrite to nitrate, respectively, in the nitrification process.

8. Scientific Research: Microorganisms serve as model organisms in various research fields.

9. Production of Vitamins and Supplements: Certain microbes are employed to produce vitamins and other bioactive compounds.

  • Propionibacterium freudenreichii: Used in the production of vitamin B12.

10. Natural Decomposers: Microorganisms play a vital role in ecosystems by decomposing dead organic matter, recycling nutrients, and maintaining ecological balance.

In conclusion, beneficial microorganisms are indispensable in myriad sectors, from healthcare to agriculture to industry. Their diverse applications underscore their significance in shaping human civilization and the natural environment.

Applications of Microorganisms

Microorganisms, due to their versatility and adaptability, have been harnessed for a myriad of applications across various sectors. From food production to scientific research, these microscopic entities have proven indispensable.

1. Food Production: Microorganisms play a pivotal role in the fermentation process, aiding in the production of diverse food items such as yogurt, cheese, and alcoholic beverages. For instance:

  • Cheese: Microbial growth contributes to the ripening and flavor of cheese. Lactobacillus Bulgaricus is a notable microbe in dairy production.
  • Alcoholic Beverages: Yeasts, such as Saccharomyces cerevisiae, facilitate the conversion of sugars into alcohol. Other specialized microbes are employed for specific beverages, like molds for sake.
  • Vinegar: Bacteria like Acetobacter Aceti are harnessed to transform alcohol into acetic acid, imparting vinegar its characteristic taste.

2. Water Treatment: Wastewater treatment plants predominantly rely on microorganisms to oxidize organic contaminants. These processes can be aerobic, as seen in sand filters, or anaerobic, where methanogens produce methane.

3. Energy Production: Microorganisms are at the forefront of biofuel production. Fermentation processes yield ethanol, while biogas reactors produce methane. Research is also underway to utilize algae for liquid fuels and bacteria to convert waste into fuel.

4. Chemicals and Enzymes: Microorganisms are instrumental in producing an array of chemicals and enzymes. Examples include:

  • Organic acids like lactic acid by Lactobacillus and citric acid by Aspergillus niger.
  • Bioactive molecules such as Streptokinase from Streptococcus and statins from Monascus purpureus.

5. Scientific Research: Microorganisms serve as invaluable tools in biotechnology and molecular biology. Yeasts, like Saccharomyces cerevisiae, are employed as model organisms due to their eukaryotic nature and ease of manipulation. They aid in genetics and proteomics research, offering insights into complex biological processes.

6. Warfare and Bioterrorism: Historically, microorganisms have been weaponized for warfare. In medieval times, diseased corpses were catapulted into enemy territories. Modern instances include bioterror attacks, such as the 1984 Rajneeshee bioterror incident.

7. Soil Health: Microbes play a crucial role in soil ecology, aiding in nutrient availability, growth stimulation, and immune system enhancement of plants. A diverse microbial community in soil often correlates with reduced plant diseases and improved yield.

In conclusion, microorganisms, with their diverse functionalities, have been integrated into various sectors, showcasing their importance in shaping human civilization and advancing scientific knowledge. Their multifaceted applications underscore the need for continued research and understanding in the realm of microbiology.

Examples of Microorganisms

Microorganisms are diverse entities that inhabit various environments, exhibiting a range of interactions with their hosts and surroundings. This article presents a selection of microorganisms, elucidating their characteristics and significance in the broader context of human health and ecology.

  1. Streptococcus Bacteria: Belonging to the genus Streptococcus, these bacteria are known to induce several illnesses in humans. While they are notorious for causing strep throat, they can also lead to conditions such as scarlet fever and, in rare instances, skin and muscle infections. Their pathogenic nature underscores the need for caution and awareness.
  2. Plasmodium – The Malaria Parasite: Malaria, a grave health concern globally, is attributed to the Plasmodium parasite. This eukaryotic microorganism is transmitted via mosquito vectors and undergoes a complex life cycle within the host. The symptoms it induces, ranging from fever to hemolytic anemia, highlight its virulence and the challenges it poses to public health.
  3. Lactobacillus rhamnosus: In contrast to the pathogenic microorganisms, Lactobacillus rhamnosus stands as a testament to the beneficial bacteria that coexist with humans. Residing in the human gut, this bacterium potentially aids in warding off gastrointestinal ailments. Its presence in dietary supplements, such as probiotics and certain yogurts, emphasizes its perceived health benefits.
  4. Cyanobacteria: Tracing their lineage to the early epochs of life on Earth, cyanobacteria have been instrumental in shaping the planet’s atmosphere. These photosynthetic bacteria are adept at converting carbon dioxide into oxygen and assimilating inorganic nitrogen into organic forms. Their role in nitrogen fixation and oxygen production underscores their ecological significance, marking them as indispensable components of the biosphere.

In summation, the realm of microorganisms is vast and varied, encompassing entities that both challenge and benefit humanity. Their intricate roles in health and ecology necessitate continued research and understanding.


Which of the following is NOT a type of microorganism?
a) Fungi
b) Virus
c) Algae
d) Fern

Which microorganism is responsible for causing tuberculosis?
a) Escherichia coli
b) Mycobacterium tuberculosis
c) Staphylococcus aureus
d) Candida albicans

Yeasts are used in the production of:
a) Cheese
b) Wine
c) Vinegar
d) Citric acid

Which of the following diseases is caused by a protozoan?
a) Malaria
b) Tuberculosis
c) Influenza
d) Measles

The process by which bacteria convert nitrogen gas from the atmosphere into ammonia is called:
a) Nitrification
b) Denitrification
c) Nitrogen fixation
d) Ammonification

Which microorganism is used to make bread rise?
a) Algae
b) Mold
c) Yeast
d) Protozoa

Penicillin, an antibiotic, is produced by:
a) Bacteria
b) Virus
c) Fungi
d) Algae

Which of the following is a beneficial bacterium found in the human gut?
a) Lactobacillus
b) Salmonella
c) Clostridium botulinum
d) Mycobacterium leprae

Which microorganism is responsible for converting milk into yogurt?
a) Aspergillus
b) Lactobacillus
c) Saccharomyces
d) Rhizobium

The study of microorganisms is called:
a) Botany
b) Zoology
c) Microbiology
d) Mycology


What are microorganisms?

Microorganisms, also known as microbes, are tiny living organisms that are usually invisible to the naked eye. They include bacteria, viruses, fungi, protozoa, and algae.

Where can microorganisms be found?

Microorganisms are ubiquitous and can be found in various environments, including soil, water, air, extreme temperatures, and inside the bodies of humans and animals.

Are all microorganisms harmful?

No, while some microorganisms can cause diseases, many are beneficial and play essential roles in processes like fermentation, decomposition, and nitrogen fixation.

What are the benefits of microorganisms?

Microorganisms are crucial for food production (e.g., yogurt, cheese, beer), waste decomposition, nutrient cycling, and even in human health by maintaining gut flora balance.

How do microorganisms reproduce?

Microorganisms reproduce through various methods. Bacteria typically reproduce asexually by binary fission, while fungi can reproduce both sexually and asexually through spores. Viruses replicate by infecting host cells.

What is the role of microorganisms in the food industry?

Microorganisms are used in the fermentation process to produce foods like yogurt, cheese, bread, and alcoholic beverages. They also help in preserving certain foods and enhancing their flavors.

How do microorganisms affect human health?

While pathogenic microorganisms can cause diseases, beneficial microbes in our gut help in digestion, vitamin synthesis, and protection against harmful microbes.

What are antibiotics, and how do they work?

Antibiotics are drugs that kill or inhibit the growth of bacteria. They target specific bacterial functions or structures, such as cell walls or protein synthesis, without harming human cells.

What is antibiotic resistance?

Antibiotic resistance occurs when bacteria evolve mechanisms to withstand the effects of antibiotics. Overuse or misuse of antibiotics can accelerate this process, leading to the emergence of “superbugs” that are hard to treat.

How can the spread of infectious diseases caused by microorganisms be prevented?

Maintaining good hygiene, getting vaccinated, safe food handling, using antibiotics judiciously, and public health measures can help prevent the spread of infectious diseases caused by microorganisms.

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Why do Laboratory incubators need CO2? What is Karyotyping? What are the scope of Microbiology? What is DNA Library? What is Simple Staining? What is Negative Staining? What is Western Blot? What are Transgenic Plants? Breakthrough Discovery: Crystal Cells in Fruit Flies Key to Oxygen Transport What is Northern Blotting?
Why do Laboratory incubators need CO2? What is Karyotyping? What are the scope of Microbiology? What is DNA Library? What is Simple Staining? What is Negative Staining? What is Western Blot? What are Transgenic Plants? Breakthrough Discovery: Crystal Cells in Fruit Flies Key to Oxygen Transport What is Northern Blotting?
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