1.3 Types of Microorganisms - Microbiology | OpenStax (2023)

Learning Objectives

By the end of this section, you will be able to:

  • List the various types of microorganisms and describe their defining characteristics
  • Give examples of different types of cellular and viral microorganisms and infectious agents
  • Describe the similarities and differences between archaea and bacteria
  • Provide an overview of the field of microbiology

Most microbes are unicellular and small enough that they require artificial magnification to be seen. However, there are some unicellular microbes that are visible to the naked eye, and some multicellular organisms that are microscopic. An object must measure about 100 micrometers (µm) to be visible without a microscope, but most microorganisms are many times smaller than that. For some perspective, consider that a typical animal cell measures roughly 10 µm across but is still microscopic. Bacterial cells are typically about 1 µm, and viruses can be 10 times smaller than bacteria (Figure 1.12). See Table 1.1 for units of length used in microbiology.

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Figure 1.12 The relative sizes of various microscopic and nonmicroscopic objects. Note that a typical virus measures about 100 nm, 10 times smaller than a typical bacterium (~1 µm), which is at least 10 times smaller than a typical plant or animal cell (~10–100 µm). An object must measure about 100 µm to be visible without a microscope.

Units of Length Commonly Used in Microbiology
Metric UnitMeaning of PrefixMetric Equivalent
meter (m)1 m = 100 m
decimeter (dm)1/101 dm = 0.1 m = 10−1 m
centimeter (cm)1/1001 cm = 0.01 m = 10−2 m
millimeter (mm)1/10001 mm = 0.001 m = 10−3 m
micrometer (μm)1/1,000,0001 μm = 0.000001 m = 10−6 m
nanometer (nm)1/1,000,000,0001 nm = 0.000000001 m = 10−9 m

Table 1.1

Microorganisms differ from each other not only in size, but also in structure, habitat, metabolism, and many other characteristics. While we typically think of microorganisms as being unicellular, there are also many multicellular organisms that are too small to be seen without a microscope. Some microbes, such as viruses, are even acellular (not composed of cells).

Microorganisms are found in each of the three domains of life: Archaea, Bacteria, and Eukarya. Microbes within the domains Bacteria and Archaea are all prokaryotes (their cells lack a nucleus), whereas microbes in the domain Eukarya are eukaryotes (their cells have a nucleus). Some microorganisms, such as viruses, do not fall within any of the three domains of life. In this section, we will briefly introduce each of the broad groups of microbes. Later chapters will go into greater depth about the diverse species within each group.

Link to Learning

How big is a bacterium or a virus compared to other objects? Check out this interactive website to get a feel for the scale of different microorganisms.

Prokaryotic Microorganisms

Bacteria are found in nearly every habitat on earth, including within and on humans. Most bacteria are harmless or helpful, but some are pathogens, causing disease in humans and other animals. Bacteria are prokaryotic because their genetic material (DNA) is not housed within a true nucleus. Most bacteria have cell walls that contain peptidoglycan.

Bacteria are often described in terms of their general shape. Common shapes include spherical (coccus), rod-shaped (bacillus), or curved (spirillum, spirochete, or vibrio). Figure 1.13 shows examples of these shapes.

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Figure 1.13 Common bacterial shapes. Note how coccobacillus is a combination of spherical (coccus) and rod-shaped (bacillus). (credit “Coccus”: modification of work by Janice Haney Carr, Centers for Disease Control and Prevention; credit “Coccobacillus”: modification of work by Janice Carr, Centers for Disease Control and Prevention; credit “Spirochete”: Centers for Disease Control and Prevention)

They have a wide range of metabolic capabilities and can grow in a variety of environments, using different combinations of nutrients. Some bacteria are photosynthetic, such as oxygenic cyanobacteria and anoxygenic green sulfur and green nonsulfur bacteria; these bacteria use energy derived from sunlight, and fix carbon dioxide for growth. Other types of bacteria are nonphotosynthetic, obtaining their energy from organic or inorganic compounds in their environment.

Archaea are also unicellular prokaryotic organisms. Archaea and bacteria have different evolutionary histories, as well as significant differences in genetics, metabolic pathways, and the composition of their cell walls and membranes. Unlike most bacteria, archaeal cell walls do not contain peptidoglycan, but their cell walls are often composed of a similar substance called pseudopeptidoglycan. Like bacteria, archaea are found in nearly every habitat on earth, even extreme environments that are very cold, very hot, very basic, or very acidic (Figure 1.14). Some archaea live in the human body, but none have been shown to be human pathogens.

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Figure 1.14 Some archaea live in extreme environments, such as the Morning Glory pool, a hot spring in Yellowstone National Park. The color differences in the pool result from the different communities of microbes that are able to thrive at various water temperatures.

Check Your Understanding

  • What are the two main types of prokaryotic organisms?
  • Name some of the defining characteristics of each type.

Eukaryotic Microorganisms

The domain Eukarya contains all eukaryotes, including uni- or multicellular eukaryotes such as protists, fungi, plants, and animals. The major defining characteristic of eukaryotes is that their cells contain a nucleus.


Protists are an informal grouping of eukaryotes that are not plants, animals, or fungi. Some algae are protists and others are bacteria; all protozoa are examples of protists.

Algae (singular: alga) are mostly made up of protists that can be either unicellular or multicellular and vary widely in size, appearance, and habitat (Figure 1.15). Algal protists are surrounded by cell walls made of cellulose, a type of carbohydrate. Algae are photosynthetic organisms that extract energy from the sun and release oxygen and carbohydrates into their environment. Cyanobacteria, a type of bacteria, is also considered an algae, but these organisms are bacterial prokaryotes and therefore have a peptidoglycan-based cell wall, unlike the cellulose-based cell wall of the algal protists. Because other organisms can use the waste products of all algae for energy, algae are important parts of many ecosystems. Many consumer products contain ingredients derived from algae, such as carrageenan or alginic acid, which are found in some brands of ice cream, salad dressing, beverages, lipstick, and toothpaste. A derivative of algae also plays a prominent role in the microbiology laboratory. Agar, a gel derived from algae, can be mixed with various nutrients and used to grow microorganisms in a Petri dish. Algae are also being developed as a possible source for biofuels.

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Figure 1.15 Assorted diatoms, a kind of algae, live in annual sea ice in McMurdo Sound, Antarctica. Diatoms range in size from 2 μm to 200 μm and are visualized here using light microscopy. (credit: modification of work by National Oceanic and Atmospheric Administration)

Protozoa (singular: protozoan) are protists that make up the backbone of many food webs by providing nutrients for other organisms. Protozoa are very diverse. Some protozoa move with help from hair-like structures called cilia or whip-like structures called flagella. Others extend part of their cell membrane and cytoplasm to propel themselves forward. These cytoplasmic extensions are called pseudopods (“false feet”). Some protozoa are photosynthetic; others feed on organic material. Some are free-living, whereas others are parasitic, only able to survive by extracting nutrients from a host organism. Most protozoa are harmless, but some are pathogens that can cause disease in animals or humans (Figure 1.16).

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Figure 1.16 Giardia lamblia, an intestinal protozoan parasite that infects humans and other mammals, causing severe diarrhea. (credit: modification of work by Centers for Disease Control and Prevention)


Fungi (singular: fungus) are also eukaryotes. Some multicellular fungi, such as mushrooms, resemble plants, but they are actually quite different. Fungi are not photosynthetic, and their cell walls are usually made out of chitin rather than cellulose.

Unicellular fungi—yeasts—are included within the study of microbiology. There are more than 1000 known species. Yeasts are found in many different environments, from the deep sea to the human navel. Some yeasts have beneficial uses, such as causing bread to rise and beverages to ferment; but yeasts can also cause food to spoil. Some even cause diseases, such as vaginal yeast infections and oral thrush (Figure 1.17).

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Figure 1.17 Candida albicans is a unicellular fungus, or yeast. It is the causative agent of vaginal yeast infections as well as oral thrush, a yeast infection of the mouth that commonly afflicts infants. C. albicans has a morphology similar to that of coccus bacteria; however, yeast is a eukaryotic organism (note the nuclei) and is much larger. (credit: modification of work by Centers for Disease Control and Prevention)

Other fungi of interest to microbiologists are multicellular organisms called molds. Molds are made up of long filaments that form visible colonies (Figure 1.18). Molds are found in many different environments, from soil to rotting food to dank bathroom corners. Molds play a critical role in the decomposition of dead plants and animals. Some molds can cause allergies, and others produce disease-causing metabolites called mycotoxins. Molds have been used to make pharmaceuticals, including penicillin, which is one of the most commonly prescribed antibiotics, and cyclosporine, used to prevent organ rejection following a transplant.

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Figure 1.18 Large colonies of microscopic fungi can often be observed with the naked eye, as seen on the surface of these moldy oranges.

Check Your Understanding

  • Name two types of protists and two types of fungi.
  • Name some of the defining characteristics of each type.


Multicellular parasitic worms called helminths are not technically microorganisms, as most are large enough to see without a microscope. However, these worms fall within the field of microbiology because diseases caused by helminths involve microscopic eggs and larvae. One example of a helminth is the guinea worm, or Dracunculus medinensis, which causes dizziness, vomiting, diarrhea, and painful ulcers on the legs and feet when the worm works its way out of the skin (Figure 1.19). Infection typically occurs after a person drinks water containing water fleas infected by guinea-worm larvae. In the mid-1980s, there were an estimated 3.5 million cases of guinea-worm disease, but the disease has been largely eradicated. In 2014, there were only 126 cases reported, thanks to the coordinated efforts of the World Health Organization (WHO) and other groups committed to improvements in drinking water sanitation.1112

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Figure 1.19 (a) The beef tapeworm, Taenia saginata, infects both cattle and humans. T. saginata eggs are microscopic (around 50 µm), but adult worms like the one shown here can reach 4–10 m, taking up residence in the digestive system. (b) An adult guinea worm, Dracunculus medinensis, is removed through a lesion in the patient’s skin by winding it around a matchstick. (credit a, b: modification of work by Centers for Disease Control and Prevention)


Viruses are acellular microorganisms, which means they are not composed of cells. Essentially, a virus consists of proteins and genetic material—either DNA or RNA, but never both—that are inert outside of a host organism. However, by incorporating themselves into a host cell, viruses are able to co-opt the host’s cellular mechanisms to multiply and infect other hosts.

Viruses can infect all types of cells, from human cells to the cells of other microorganisms. In humans, viruses are responsible for numerous diseases, from the common cold to deadly Ebola (Figure 1.20). However, many viruses do not cause disease.

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Figure 1.20 (a) Members of the Coronavirus family can cause respiratory infections like the common cold, severe acute respiratory syndrome (SARS), and Middle East respiratory syndrome (MERS). Here they are viewed under a transmission electron microscope (TEM). (b) Ebolavirus, a member of the Filovirus family, as visualized using a TEM. (credit a: modification of work by Centers for Disease Control and Prevention; credit b: modification of work by Thomas W. Geisbert)

Check Your Understanding

  • Are helminths microorganisms? Explain why or why not.
  • How are viruses different from other microorganisms?

Microbiology as a Field of Study

Microbiology is a broad term that encompasses the study of all different types of microorganisms. But in practice, microbiologists tend to specialize in one of several subfields. For example, bacteriology is the study of bacteria; mycology is the study of fungi; protozoology is the study of protozoa; parasitology is the study of helminths and other parasites; and virology is the study of viruses (Figure 1.21). Immunology, the study of the immune system, is often included in the study of microbiology because host–pathogen interactions are central to our understanding of infectious disease processes. Microbiologists can also specialize in certain areas of microbiology, such as clinical microbiology, environmental microbiology, applied microbiology, or food microbiology.

In this textbook, we are primarily concerned with clinical applications of microbiology, but since the various subfields of microbiology are highly interrelated, we will often discuss applications that are not strictly clinical.

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Figure 1.21 (a) A virologist samples eggs from this nest to be tested for the influenza A virus, which causes avian flu in birds. (b) A biologist performs a procedure to identify an organism that causes ulcerations in humans (credit a: U.S. Fish and Wildlife Service; credit b: James Gathany / CDC; Public Domain)

Eye on Ethics

Bioethics in Microbiology

In the 1940s, the U.S. government was looking for a solution to a medical problem: the prevalence of sexually transmitted diseases (STDs) among soldiers. Several now-infamous government-funded studies used human subjects to research common STDs and treatments. In one such study, American researchers intentionally exposed more than 1300 human subjects in Guatemala to syphilis, gonorrhea, and chancroid to determine the ability of penicillin and other antibiotics to combat these diseases. Subjects of the study included Guatemalan soldiers, prisoners, prostitutes, and psychiatric patients—none of whom were informed that they were taking part in the study. Researchers exposed subjects to STDs by various methods, from facilitating intercourse with infected prostitutes to inoculating subjects with the bacteria known to cause the diseases. This latter method involved making a small wound on the subject’s genitals or elsewhere on the body, and then putting bacteria directly into the wound.13 In 2011, a U.S. government commission tasked with investigating the experiment revealed that only some of the subjects were treated with penicillin, and 83 subjects died by 1953, likely as a result of the study.14

Unfortunately, this is one of many horrific examples of microbiology experiments that have violated basic ethical standards. Even if this study had led to a life-saving medical breakthrough (it did not), few would argue that its methods were ethically sound or morally justifiable. But not every case is so clear cut. Professionals working in clinical settings are frequently confronted with ethical dilemmas, such as working with patients who decline a vaccine or life-saving blood transfusion. These are just two examples of life-and-death decisions that may intersect with the religious and philosophical beliefs of both the patient and the health-care professional.

No matter how noble the goal, microbiology studies and clinical practice must be guided by a certain set of ethical principles. Studies must be done with integrity. Patients and research subjects provide informed consent (not only agreeing to be treated or studied but demonstrating an understanding of the purpose of the study and any risks involved). Patients’ rights must be respected. Procedures must be approved by an institutional review board. When working with patients, accurate record-keeping, honest communication, and confidentiality are paramount. Animals used for research must be treated humanely, and all protocols must be approved by an institutional animal care and use committee. These are just a few of the ethical principles explored in the Eye on Ethics boxes throughout this book.

Clinical Focus


Cora’s CSF samples show no signs of inflammation or infection, as would be expected with a viral infection. However, there is a high concentration of a particular protein, 14-3-3 protein, in her CSF. An electroencephalogram (EEG) of her brain function is also abnormal. The EEG resembles that of a patient with a neurodegenerative disease like Alzheimer’s or Huntington’s, but Cora’s rapid cognitive decline is not consistent with either of these. Instead, her doctor concludes that Cora has Creutzfeldt-Jakob disease (CJD), a type of transmissible spongiform encephalopathy (TSE).

CJD is an extremely rare disease, with only about 300 cases in the United States each year. It is not caused by a bacterium, fungus, or virus, but rather by prions—which do not fit neatly into any particular category of microbe. Like viruses, prions are not found on the tree of life because they are acellular. Prions are extremely small, about one-tenth the size of a typical virus. They contain no genetic material and are composed solely of a type of abnormal protein.

CJD can have several different causes. It can be acquired through exposure to the brain or nervous-system tissue of an infected person or animal. Consuming meat from an infected animal is one way such exposure can occur. There have also been rare cases of exposure to CJD through contact with contaminated surgical equipment15 and from cornea and growth-hormone donors who unknowingly had CJD.1617 In rare cases, the disease results from a specific genetic mutation that can sometimes be hereditary. However, in approximately 85% of patients with CJD, the cause of the disease is spontaneous (or sporadic) and has no identifiable cause.18 Based on her symptoms and their rapid progression, Cora is diagnosed with sporadic CJD.

Unfortunately for Cora, CJD is a fatal disease for which there is no approved treatment. Approximately 90% of patients die within 1 year of diagnosis.19 Her doctors focus on limiting her pain and cognitive symptoms as her disease progresses. Eight months later, Cora dies. Her CJD diagnosis is confirmed with a brain autopsy.

Go back to the previous Clinical Focus box.


1.3 Types of Microorganisms - Microbiology | OpenStax? ›

Microorganisms are found in each of the three domains of life: Archaea, Bacteria, and Eukarya. Microbes within the domains Bacteria and Archaea are all prokaryotes (their cells lack a nucleus), whereas microbes in the domain Eukarya are eukaryotes (their cells have a nucleus).

What are the 3 main types of microorganisms? ›

The most common types are bacteria, viruses and fungi. There are also microbes called protozoa. These are tiny living things that are responsible for diseases such as toxoplasmosis and malaria.

What are the 1.6 classification of microorganisms? ›

Microorganisms include bacteria, archaea, algae, fungi, protozoa, etc. Although viruses are not considered living organisms, sometimes they are also included in the microorganisms category.

What are the 7 major types of microorganisms? ›

Microorganisms are divided into seven types: bacteria, archaea, protozoa, algae, fungi, viruses, and multicellular animal parasites ( helminths ). Each type has a characteristic cellular composition, morphology, mean of locomotion, and reproduction.

What are the four 4 types of microorganisms? ›

Microbial diversity is truly staggering, yet all these microbes can be grouped into five major types: Viruses, Bacteria, Archaea, Fungi, and Protists. Let's look at each one in more detail.

What are microorganisms in microbiology? ›

Main content. Technically a microorganism or microbe is an organism that is microscopic. The study of microorganisms is called microbiology. Microorganisms can be bacteria, fungi, archaea or protists. The term microorganisms does not include viruses and prions, which are generally classified as non-living.

What are different types of microorganisms? ›

The major groups of microorganisms—namely bacteria, archaea, fungi (yeasts and molds), algae, protozoa, and viruses—are summarized below. Links to the more detailed articles on each of the major groups are provided.

What are the 7 levels of classification for bacteria? ›

In prokaryotic taxonomy, the overall classification system is a hierarchic grouping. It concerns from highest to the lowest: domain, phylum, class, order, family, genus, species, and subspecies. Prokaryotic systematics is a scientific discipline that evolves with the technological tools that become available.

How do you classify microorganisms? ›

The most widely employed methods for classifying microbes are morphological characteristics, differential staining, biochemical testing, DNA fingerprinting or DNA base composition, polymerase chain reaction, and DNA chips.

What are five categories of micro organisms? ›

There are five different categories of microorganisms—bacteria, algae, protozoa, fungi, and viruses—explained in further detail in Mystery Microorganisms. Microorganisms cover almost all the kingdoms of life.

What are the 8 types of microbes? ›

There are several types of microbes, which include bacteria, archaea, protozoa, fungi, algae, lichens, slime molds, viruses, and prions.

What are the top 10 microorganisms? ›

Top 10 Microorganisms for Educating Students
  1. Staphylococcus aureus.
  2. Staphyloccocus epidermidis.
  3. Escherichia coli.
  4. Proteus mirabilis.
  5. Mycobacterium smegmatis.
  6. Pseudomonas aeruginosa.
  7. Candida albicans.
  8. Peniciliium rubens.
Jul 13, 2017

What are the 6 main types of microorganisms? ›

There are six types of microorganisms (bacteria, archaea, protozoa, algae, fungi, and viruses) but we will focus on the main three: bacteria, fungi and viruses. Explain the differences between the three types. Helpful and harmful microorganisms.

What are the 4 main types of microorganisms that cause disease? ›

  • Bacteria. These one-cell organisms are responsible for illnesses such as strep throat, urinary tract infections and tuberculosis.
  • Viruses. Even smaller than bacteria, viruses cause a multitude of diseases ranging from the common cold to AIDS.
  • Fungi. ...
  • Parasites.

What are microbes Class 12? ›

Microbes are minute, unicellular organisms that are invisible to the naked eye. They are also known as microorganisms or microscopic organisms as they could only be seen under a microscope. They make up almost 60% of the earth's living matter.

What is microorganism class 8? ›

Microorganisms- A living organism that is very minute to be seen by the naked eye, especially a single celled organism, such as a bacterium is called microorganism. Microorganisms are classified into four major groups mainly bacteria, fungi, protozoa and some algae.

What are the major microorganisms? ›

Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa and some algae.

What 3 features are common to all microorganisms? ›

Key Points
  • Bacteria vary from species to species, thus assigning many common traits to bacteria is difficult. ...
  • There are three notable common traits of bacteria, 1) lack of membrane-bound organelles, 2) unicellular and 3) small (usually microscopic) size.
Dec 24, 2022

What are the three 3 major types of microorganisms that are associated with food? ›

Microbes such as bacteria, molds, and yeasts are employed for the foods production and food ingredients such as production of wine, beer, bakery, and dairy products.

What are the 3 ways which microorganisms are usually spread? ›

Five common ways germs are spread:
  • Nose, mouth, or eyes to hands to others: Germs can spread to the hands by sneezing, coughing, or rubbing the eyes and then can be transferred to other family members or friends. ...
  • Hands to food: ...
  • Food to hands to food: ...
  • Infected child to hands to other children: ...
  • Animals to people:
Oct 4, 2022

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