Take a moment to observe the natural world around you. Each day, you likely encounter many humans and non-human animals, such as birds, insects, lizards, and rodents. However, the natural world that is visible to us represents only a small fraction of the whole. 

Microorganisms Microorganisms

Create learning materials about Microorganisms with our free learning app!

  • Instand access to millions of learning materials
  • Flashcards, notes, mock-exams and more
  • Everything you need to ace your exams
Create a free account
Table of contents

    Many organisms on our planet are too small for humans to see. These numerous tiny creatures are known as microorganisms.

    In the following article, we will:

    • define microorganisms
    • discuss how they can be both harmful and beneficial
    • look at their different types
    • and, lastly, look at some examples of each.

    What are microorganisms?

    Let's start by looking at the definition of a microorganism.

    A microorganism is any living organism that is too small to be visible to the naked eye, meaning they are microscopic and are only observable under a microscope.

    Microorganisms were long suspected to exist, but their formal discovery was made in 1673 when Dutch scientist Antoine Philips van Leeuwenhoek confirmed their existence using an early microscope consisting of only a single lens.

    "I then most always saw, with great wonder, that in the said matter there were many very little living animalcules, very prettily a-moving." - Leeuwenhoek.

    This makes Leeuwenhoek one of the two "fathers of microbiology" along with Louis Pasteur, who also made numerous discoveries, including pasteurization.

    Microbiology is the study of microorganisms.

    Pasteurization refers to the heat treatment of food products to kill pathogens and delay spoilage.

    Characteristics of microorganisms

    Microorganisms are impressively diverse, some of their main characteristics are:

    • They include single- and multi-celled organisms, both prokaryotes and eukaryotes can be microorganisms. They can be bacteria, archaea, fungi, or protists.
    • We can only see up to 100 micrometres (µm) with our bare eyes, so organisms smaller than this size are considered microorganisms (Fig. 1).
    • Microorganisms are found in every ecosystem and can also be closely associated with many multicellular organisms.

    Viruses are non-living. Thus, they are not really "microorganisms" although they are included in these and studied as part of microbiology.

    Microorganisms, Diagram showing the size of microorganisms StudySmarterFig. 1: Microorganisms come in a range of sizes, yet none of them are visible with just the naked eye. A bacteria can have a size of about 1 μm.

    Microorganisms and their function

    Microorganisms serve a wide variety of functions, depending on their type and their location. Some of them, like bacteria, can play beneficial and harmful roles inside other living organisms, depending on species and strain. But they also play a vital role in the maintenance of the environment.

    Let's have a closer look at some microorganisms' functions:

    1. Decomposition: microorganisms break down dead organic matter such as plant material and animal remains, releasing nutrients back into the soil. Microorganisms also play a crucial role in nutrient cycling: they take part in the nitrogen, phosphorus, and sulfur biogeochemical cycles. They help to convert these nutrients into forms that can be used by plants and other organisms.

    2. Oxygen production: protists like microscopic algae are a vital source of oxygen for our planet. It is estimated that over 50% of our oxygen is available due to the conversion of carbon dioxide to oxygen by microscopic algae!

    3. Fermentation: microorganisms such as yeast and bacteria are used in the production of many foods and beverages, including bread, cheese, beer, and yoghurt.

    4. Bioremediation: some microorganisms have the ability to break down toxic pollutants in the environment, such as oil spills or industrial waste. This process, known as bioremediation, is a current interesting area of reascan help to clean up contaminated sites and restore ecosystems.

    Types of microorganisms

    As stated previously, microorganisms are generally considered to be bacteria, archaea, fungi, and protists (Fig. 2).

    Microorganisms Types of prokaryotic and eukaryotic microoganisms StudySmarterFig. 2: Microorganisms include prokaryotes (bacteria and archaea) and eukaryotes (protists and fungi). The archaea are not depicted in this image.


    Bacteria are single-celled prokaryotic microorganisms that can live freely or in association with a host and are omnipresent within our environment. When bacteria are said to be omnipresent, it means that they are present in virtually every habitat on Earth, from the soil to the ocean to within our bodies. Bacteria even inhabit seemingly inhospitable environments, such as within the hot soil in and near volcanoes and in radioactive waste.

    Remember, since they are prokaryotes, they do not have a membrane-bound nucleus. You can go over these differences in our article Eukaryotic and Prokaryotic Cells.

    Bacteria play a vital role in our environment and within our bodies. The presence of bacteria is essential for all other life on Earth.

    • Bacteria are both decomposers and producers - they decompose dead organic matter and waste into inorganic matter rich in nutrients, thus allowing plants to grow. These plants then feed herbivores, which in turn feed carnivores.
    • Inside and on animals, including humans, bacteria can be found as part of the microbiome, playing vital roles such as aiding digestion. In fact, disruption of the microbiome is believed to play a role in developing specific syndromes and diseases.
      • While most bacteria are harmless or beneficial to humans, some can cause illness, disease, and even death. These are known as pathogenic bacteria.

    Microorganisms Micrograph of pathogenic bacteria Staphylococcus aureus StudySmarter

    Fig. 3: Scanning electron micrograph of methicillin-resistant Staphylococcus aureus bacteria (yellow) and a dead human white blood cell (red).

    Throughout human history, some of these pathogenic bacterial species have been responsible for human mortality on a massive scale. This was particularly true during the pre-antibiotic era when diseases like tuberculosis and plague killed millions.

    Although antibiotics have made it possible to better control many of these diseases (and even to eradicate them from many countries), antibiotic-resistant strains of bacteria are becoming increasingly common and concerning. Currently, around 700,000 people are killed by antibiotic-resistant bacteria annually worldwide, but this number could reach as high as 10 million annually by 2050 if new antibiotics are not developed!

    Want to learn more about bacterial pathogens? Check out "Bacteria"!


    Archaea are another type of single-celled prokaryotic organism and, until relatively recently, were believed to be another kind of bacteria known as archaebacteria. This is due to the many similarities that archaea and bacteria share.

    In addition to being single-celled prokaryotes, archaea are also believed to be some of the earliest living organisms. They can survive in extreme environments, such as deep-sea hydrothermal vents that may exceed 212° F!

    It was in 1977 that two scientists named George Fox and Carl Woese could definitively separate archaea into its own domain!

    So, what are the differences between the two? Well, archaea tend to have genes that more closely resemble those of eukaryotic organisms, and archaea have cell walls that do not contain peptidoglycan, a polysaccharide that forms the cell wall in bacterial species.

    Unlike bacteria, archaea are also not currently known to be pathogenic, meaning there are no diseases that are known to be caused by archaea, at least not yet.

    Archaea are the only known methanogens, living organisms producing their own methane via internal chemical reactions. These methanogens can be found in the environment, such as in wetlands where they produce swamp gasses, as well as within the digestive systems of many mammals, including humans.

    Archaea also play an essential role in the nitrogen cycle through the oxidation of ammonia, which is critical for the cycling of nutrients in most ecosystems.

    Microorganisms Example of Archaea in a hot spring StudySmarter

    Fig. 4: The Morning Glory hot spring in Yellowstone National Park obtains its colouration from archaea. Source: Wiki Commons, Public Domain


    Fungi are eukaryotic organisms that can be microscopic or visible. Perhaps the fungi you are most familiar with are mushrooms since they are visible to the naked eye (and, sometimes, edible)!

    Despite the physical appearance of mushrooms, fungi are not plants and are, instead, members of a separate biological kingdom unique from animals and plants. For our purposes, we are only interested in microscopic fungi, known as microfungi.

    Microfungi include molds, mildews, rusts, and yeasts. Like bacteria and archaea, fungi exist in most ecosystems and within many organisms.

    Most microfungi have a hyphal structure, which can be observed as the white, fuzzy mold growing on old food products.

    While large, visible fungal species (mushrooms) are essential in many human cuisines, microfungi also play many important roles. Microfungi of the Penicillium genus, for example, were used by Scottish microbiologist Sir Alexander Fleming to create the world's first antibiotic - penicillin! This discovery revolutionized medicine and saved countless lives.

    Some species of Penicillium are also used in culinary practices, specifically in creating blue cheese! For those of you who are vegetarian, the microfungi species Fusarium venenatum is used to create the mycoprotein "meats" sold by the brand called Quorn.

    Unfortunately, some microfungi species can be dangerous to humans. Among these pathogenic microfungi are those that cause benign conditions like athlete's foot and yeast infections. However, other conditions can be fatal, such as mucormycosis, caused by a black fungus of the order Mucorales. Infections by pathogenic microfungi are treated with antifungals.

    Microorganisms A penicillin poster from World War 2 StudySmarterFig. 5: A penicillin poster from World War II. Penicillin was the first antibiotic ever developed and saved many lives. Source: Wiki Commons, Public Domain


    Protists are mostly microscopic, single-celled organisms, including algae species, amoebas, ciliates, slime molds, and more. Protists appear to be a very diverse group of organisms, and protist taxonomy is mainly in a state of flux due to continually changing classifications and discoveries.

    Protists were once considered a part of their own kingdom, known as Protista, but were ill-defined and are generally considered to be any eukaryote that does not fit within the animal, fungal, or plant kingdoms.

    Protists can be autotrophic, producing their own energy through photosynthesis, or heterotrophic, consuming other organisms to meet their energy requirements.

    Despite the changing classifications and general lack of clarity regarding protist taxonomy, protist species are of significant interest to humans for positive and negative reasons.

    For example, some species of protist feed upon carbon dioxide producing bacteria. In doing so, these protists help to mitigate greenhouse gas emissions and may be one of the key tools in combatting anthropogenic climate change!

    Some protists are also of medical significance to humans, as they can cause common diseases, such as giardiasis (Fig. 7) and malaria. Malaria is caused by protists of the Plasmodium genus carried by mosquitoes of the Anopheles genus. Nearly 250 million people, mostly in the developed world, are infected with malaria annually, resulting in over 600,000 deaths.

    Microorganisms A protozoan, a type of microorganism StudySmarter

    Fig. 6: Scanning electron micrograph of the protozoan Giardia lamblia during cell division. The two daughter organisms will become separate individuals when division is finished.

    Examples of microorganisms

    Lastly, let's look at specific examples of microorganisms from each type - bacteria, archaea, fungi, and protists. First up, let's look at a few of the more well-known pathogenic bacteria.

    • Escherichia coli: Commonly referred to as E. coli, most strains are harmless residents of our bodies, but some are pathogenic and can cause severe diarrheal illness.

    • Mycobacterium tuberculosis: The causative agent of tuberculosis, which usually causes pulmonary disease, but can affect many parts of the body, including the lymph nodes and the gastrointestinal tract.

    • Borrelia burgdorferi: The causative agent of Lyme disease, which is transmitted to humans via bites from infected ticks.

    • Vibrio cholerae: The causative agent in cholera, which is a potentially life-threatening diarrheal disease most common in developing countries.

    • Staphylococcus aureus: While often harmless (around 30% of humans carry it on their skin), it can be pathogenic and can cause a wide variety of illnesses, from pneumonia to sepsis.

    Next, let's look at some examples of archaea species.

    • Methanobrevibacter smithii: archaea species responsible for producing methane in human flatulence! M. smithii plays a role in the digestion of polysaccharides within the human gut.

    • Saccharolobus solfataricus: This species is known to be an "extremophile", which means it can survive in "extreme" environments. It was first identified within the Solfatara volcano in Italy (1980).

    • Nitrosopumilus maritimus: This species plays an essential role in ammonia oxidation and is abundant in seawater.

    The following are some notable species of microfungi.

    • Candida albicans: Though often a harmless commensal fungus of the human gastrointestinal tract, it can also be pathogenic and is the most common cause of yeast infections and oral thrush.

    • C. auris: This species of Candida can cause severe and life-threatening infections in humans. C. auris is an emerging public health concern, particularly given that it is resistant to most antifungals.

    • Penicillium chrysogenum: This is the Penicillium species complex that is used to create the antibiotic penicillin.

    Lastly, the following is a notable species of protist.

    • Giardia duodenalis or lamblia: The protist species responsible for giardiasis, a common parasitic illness characterized by gastrointestinal symptoms, principally diarrhea and abdominal pain. G. duodenalis is typically spread via contaminated food or water.

    Now, I hope that you feel more confident in your understanding of microorganisms!

    Microorganisms - Key takeaways

    • A microorganism is any living organism that is too small to be visible to the naked eye. There are generally considered to be four types of microorganisms- bacteria, archaea, fungi, and protists.
    • Bacteria are single-celled prokaryotic microorganisms that are both decomposers and producers - they decompose dead organic matter and waste into inorganic matter.
    • Archaea are another type of single-celled prokaryotic organism. Archaea are the only known methanogens, which are living organisms that produce their own methane via internal chemical reactions.
    • Fungi are eukaryotic organisms that can be microscopic or visible. Microscopic fungi are known as microfungi and include molds, mildews, rusts, and yeasts.
    • Protists were once considered a part of their own kingdom, known as Protista, but are now ill-defined and are mainly considered to be any eukaryote that does not fit within the animal, fungal or plant kingdoms.


    1. Fig. 3: Methicillin-resistant Staph bacteria (https://www.flickr.com/photos/nihgov/49234831117/in/album-72157656657569008/) by NIH Image Gallery, Public domain, Flickr.com.
    2. Fig. 6: Giardia lamblia in a late stage of cell division (https://upload.wikimedia.org/wikipedia/commons/2/21/Giardia_lamblia_in_a_late_stage_of_cell_division.jpg) by Dr. Stan Erlandsen, Public domain.
    Frequently Asked Questions about Microorganisms

    Which are the useful microorganisms?

    Many microorganisms are useful to humans. Bacteria and archaea in our gut are vital to the proper digestion of food, fungi are used to create antibiotics and foods, and protists can be used to combat climate change.

    What is a microorganism?

    A microorganism is any living organism that is too small to be visible to the naked eye, meaning they are microscopic and are only observable under some kind of microscope. 

    What are three examples of harmful microorganisms?

    Three examples of harmful microorganisms are: Mycobacterium tuberculosis is a bacterial species that causes tuberculosis. Fungi of the order Mucorales can cause fatal infections and protists of the Plasmodium genus cause malaria.

    What are microorganisms and their function?

    Microorganisms are any living organism too small to be visible to the naked eye, meaning they are microscopic. Microorganisms can serve many functions, from nutrient recycling in the environment to facilitating digestion in the intestines of animals.

    What are the 4 types of microorganisms?

    There are generally considered to be four types of microorganism- bacteria, archaea, fungi, and protists. 

    What is the importance of microorganisms in the human body?

    The importance of microorganisms in the human body is that, specifically for bacteria and archaea, they aid in the digestion of foods. Disruption of the microbiome is associated with several syndromes and diseases.

    Test your knowledge with multiple choice flashcards

    The first scientist to successfully identify microorganisms was _________________.

    True or False: Microorganisms include both single- and multi-celled organisms, which means that both prokaryotes and eukaryotes can be microorganisms.

    True or False: Viruses are microorganisms.

    About StudySmarter

    StudySmarter is a globally recognized educational technology company, offering a holistic learning platform designed for students of all ages and educational levels. Our platform provides learning support for a wide range of subjects, including STEM, Social Sciences, and Languages and also helps students to successfully master various tests and exams worldwide, such as GCSE, A Level, SAT, ACT, Abitur, and more. We offer an extensive library of learning materials, including interactive flashcards, comprehensive textbook solutions, and detailed explanations. The cutting-edge technology and tools we provide help students create their own learning materials. StudySmarter’s content is not only expert-verified but also regularly updated to ensure accuracy and relevance.

    Learn more
    StudySmarter Editorial Team

    Team Microorganisms Teachers

    • 13 minutes reading time
    • Checked by StudySmarter Editorial Team
    Save Explanation

    Study anywhere. Anytime.Across all devices.

    Sign-up for free

    Sign up to highlight and take notes. It’s 100% free.

    Join over 22 million students in learning with our StudySmarter App

    The first learning app that truly has everything you need to ace your exams in one place

    • Flashcards & Quizzes
    • AI Study Assistant
    • Study Planner
    • Mock-Exams
    • Smart Note-Taking
    Join over 22 million students in learning with our StudySmarter App