What are Microbes?

Microbes are microscopic living organisms that can only be detected with the help of specialized laboratory equipment called a microscope. These living organisms are so small that they are not visible to the naked eye.

What are the different types of microbes?

The following are the two broad categories of microbes and their subgroups:


Prokaryotes are single-celled, microscopic organisms that do not possess any intact nucleus and are devoid of organelles. These microbes contain specialized cell wall that contains protrusions. Two types of organisms fall within this category.

  • Bacteria

A bacterium is a unicellular organism that lacks membrane-bound cell organelles, including the nucleus. Bacteria can be beneficial as well as harmful to human beings. Escherichia coli dwells in the human intestine and forms a mutual relationship with humans. Only a few types of bacteria are known to cause diseases. An example of one such disease-causing bacterium is Streptococcus pneumoniae, which causes pneumonia.

The invention of antibiotics has helped to control these disease-causing microorganisms. These compounds affect the synthesis of the bacterial cell wall and inhibit protein synthesis or the synthesis of enzymes and factors required for replication and RNA synthesis in bacteria. Thus, antibiotics are either bacteriostatic (growth inhibitors) or bactericidal (kill bacteria).

Bacterial species are categorized into aerobic (Bacillus sp., Mycobacterium tuberculosis, etc.) and anaerobic (Clostridium, Propionibacterium), depending on oxygen requirement. Chemolithotrophic bacteria oxidize inorganic substrates such as hydrogen sulfide, ferrous iron, ammonia, etc., to obtain energy and facilitate maintenance of cells and biosynthesis of products required for their survival. In aerobic chemolithotrophs, oxygen acts as the electron acceptor, and the inorganic substrate serves as the electron donor. In anaerobic chemolithotrophs, various organic and inorganic molecules serve as electron acceptors.

A diagram shows the structure of a bacterial cell. An ovoid shaped open structure is shown surrounded by several finger-like projections labeled as cilia. Two larger projections present at the top right side of the image are labeled as flagella. Within the open ovoid shape, another open ovoid structure is present. This contains interconnected thread-like structure labeled as chromosome is present at the center. Several dotted structures are present scattered and one is labeled as ribosome. An irregular-shaped structure labeled as plasmid is also present above the chromosome. Cytoplasm is labeled below the chromosome. The outer layer of the ovoid structure is labeled capsule. The space between the outer and inner ovoid structures is labeled cell wall and the border of the inner ovoid structure is labeled cell membrane.
Structure of a bacterial cell
  • Archaea

Archaea are minute living organisms that differ from bacteria in terms of molecular characteristics. These organisms consist of a cell membrane made up of only a single layer of lipids. The cell wall of these organisms is composed of the surface layer (S-layer). This layer is made up of paracrystalline protein. Microorganisms that are categorized as archaea are extremophiles (survive in extreme conditions such as Pompeii worm).


Eukaryotes are made up of eukaryotic cells containing a distinct nucleus and other membrane-bound organelles. Such microorganisms consist of well-defined organelles, such as mitochondria, chloroplasts, and the Golgi apparatus. There are two types of microbes that come under this group:

  • Protists

Protists are single-celled microorganisms that possess essential components called organelles that play an important role in the proper functioning of the cell. Algae and many parasites comprise this category of microorganisms. They can use special locomotory components, such as flagella and cilia. Trypanosoma is a type of protist that acts as a parasite. Trypanosoma brucei causes sleeping sickness disease in humans.

  • Fungi

Fungus is considered different from living organisms, and these microorganisms are placed under completely different kingdoms. This is because these microorganisms consist of an outermost layer called the cell wall, composed of chitin. Molds and yeast are examples of microorganisms in this category. Some fungi, namely mushrooms, are edible.

The most popular fungus is yeast, which has many applications in human existence. It is employed in industries to make bread and cheese by the metabolic process known as fermentation. Yeast is also responsible for converting the carbohydrates used in making dough for bread into carbon dioxide. This results in the fermentation of the dough and causes it to rise.

Apart from the baking industry, yeast is also employed in the preparation of cheese. It is responsible for providing a distinctive texture to the cheese. Yeast also confers a particular flavor to it. Apart from yeast, another type of fungus called mold is also used in the cheese-making process.

Fungi are also accountable for causing infections and diseases in humans. Diseases that are caused by fungi are called mycoses. These microorganisms can cause skin infection by destroying the tissue beneath the skin and can spread to different parts of the body and causing disease. Itching, rashes, and redness of the skin are common symptoms of mycoses.

Athlete's foot is a fungal disease in which the foot of an individual is infected by a fungal species (Tinea). Vaginal fungal infection is another fungal disease caused by Candida albicans.

A diagram shows the structure of a fungus. A filamentous structure with an oval-shaped structure at the top  is present labeled as sporangium. Another filamentous structure with four spikes at the top (two each on both sides) release minute spherical structures labeled as spores. These filamentous stalks are attached to a common base labeled food source and have root-like hyphae.
Structure of a filamentous fungi


A virus is an acellular microorganism and lies on the border of non-living and living organisms. Viruses are obligate intracellular parasites, not extracellular parasites. These microorganisms are structurally different from bacteria. They do not possess a cellular body. They are composed of a protein covering called a capsid. The genetic material of this microorganism is present within the protein covering, and the genetic information is present in the macromolecule called nucleic acid. Ribonucleic acid (RNA) or Deoxyribonucleic acid (DNA) can possess genetic information regarding a virus. This microorganism can transmit its genetic material into the host cell, leaving behind the protein covering outside the host cell.

Viruses are responsible for causing numerous infections in the human body, such as the common cold in humans. The dangerous infection called Covid-19 (coronavirus diseases 2019) has been caused by this microorganism in humans and is now a matter of concern among researchers. The human immunodeficiency virus (HIV) is another disease-causing virus that can spread amongst humans through bodily fluids, such as blood (during transfusion and by using infected needles), semen (engaging in unprotected sexual intercourse), and through vaginal fluids. This virus affects the CD4 cells or the helper T-cells, important for activating cytotoxic T-cells and B-cells. The destruction of the helper T-cells, thus, adversely affects the adaptive immune response, due to which the immune system weakens and gets damaged gradually.

The drugs that are administered to kill bacteria cannot kill viruses as antibiotics are ineffective against them. This is because viruses possess and undertake different reproductive and survival machinery and mechanisms, respectively, and lack certain enzymes or proteins targeted by antibiotics.

A diagram shows the structure of a virus. A bordered circular structure with a hexagonal-shaped protein coat is shown. The hexagonal-shaped structure is labeled as 'capsid (protein coat). This encloses the genetic material, labeled as nucleic acid, of the virus shown as a twisted helical structure. The circular structure is covered by several thick spikes labeled as spikes (projection).
Structure of a virus

Microbial nutrition

Just like other living organisms, microbes also require energy to survive. Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur are the essential nutrients.

Some of these microbes utilize organic molecules, while others utilize inorganic molecules. Microbes that utilize preformed carbon obtained from a source are called heterotrophs, whereas microbes capable of synthesizing carbon-containing compounds from inorganic sources are called autotrophs. These microbes are further categorized as:

  • Photoautotrophs utilize solar energy to synthesize carbon-containing compounds (usually carbon dioxide).
  • Chemotrophs use chemical compounds as their source of energy.

Where do microbes live?

Microbes can survive in diverse environments and are almost found everywhere; in the air, water, and soil. They can survive in extreme conditions and environments with limited water availability, high salinity, and low oxygen concentration.

Some bacterial species, in particular, can survive in harsh and extreme temperatures, such as volcanoes and hydrothermal vents deep within oceanic trenches. Such microbes are endowed with special components that allow these minute living organisms to survive in such extreme environments.

Apart from harsh environments or environments with extreme climatic conditions, microbes also thrive as parasites, either within or on the body surface of other organisms. In the case of humans, specific microbial species form a group called gut microbes. These are present in the gastrointestinal tract and form a part of the human microbiome.

Thus, microorganisms exhibit diversity in living environments.

Are all microbes harmful?

All microbes are not harmful, although some minute living organisms seriously threaten other organisms like humans. Some microbes are neutral, while others are causative agents of many dreadful diseases like malaria or dengue.

However, microorganisms can be harmful when employed as bioweapons, as witnessed in the Rajneeshee bioterror attack of 1984 and the release of anthrax by the cult Aum Shinrikyo in Tokyo in 1993.

Applications of microbes

Microorganisms are also known to act as decomposers. Certain bacterial and fungal species decompose the dead remains of plants and animals to allow nutrients to revert to nature. This allows the recycling of nutrients in the environment.

Microorganisms can be beneficial to humans in many ways.

  • They are used in the food industry as well as in the production of medicines like antibiotics. Yeast is used in the beverage industry for the preparation of alcoholic drinks.
  • Bacteria are used for genetic engineering, and plasmids present in them are used for cloning. Plasmids present in bacteria can replicate without relying on bacterial chromosomes. These are the extrachromosomal DNA present in the bacteria.
  • Lactobacillus is the microbe that is used in curd making.
  •  Microbes called methanogens are used for preparing biogas, which are utilized in households as a replacement for liquid petroleum gas (LPG) cylinders.
  • Different aerobic and anaerobic bacterial species are used in the biological treatment during wastewater treatment.

Thus, microorganisms play several important roles in human activities as well as in the activities of nature.

Common Mistakes

Students may deduce that binary fission in bacteria is the same as mitosis. However, binary fission and mitosis are two different processes, although they possess similarities. Duplication of the genetic material is a common aspect shared by both. The formation of the spindle fibers is not present in the reproduction process of bacteria.

Students may believe that all microbes are dependent on other living organisms. However, this is incorrect since all microbes are not entirely dependent on other living organisms for survival. Microscopic plants and many microbes, namely algae, can make their food and exist independently.

Students may also assume that antibiotics are helpful during a cold. However, antibiotics are compounds that are only capable of killing bacteria. These medicines cannot negatively affect microorganisms other than bacteria. Since viruses cause flu, antibiotics, in this case, would not be helpful.

Context and Applications

This topic can be useful for professional exams of higher secondary, undergraduates, and postgraduates.

  • Bachelor of Science in Zoology
  • Bachelor of Science in Microbiology
  • Master of Science in Microbiology
  • Master of Science in Zoology
  • Food Microbiology
  • Microbiomes
  • Pathogenicity and Virulence.
  • Antimicrobial Resistance

Practice Problems

Q1: Where do microbes live?

(a) Only in humid places

(b) Only in dark and moist places

(c) Extreme environments

(d) Almost everywhere

Correct choice: (d)

Q2: Which of the following is not present in bacteria?

(a) Cell membrane

(b) DNA

(c) Nucleus

(d) Cytoplasm

Correct choice: (c)

Q3: Which of the following is a fungal disease?

(a) Cholera

(b) Pneumonia

(c) Athlete's foot

(d) Common cold

Correct choice: (c)

Q4: Which of the following microbes is used in the baking industry?

(a) Streptococcus

(b) Yeast

(c) Canida albicans

(d) Lactobacillus

Correct choice: (b)

Q5: Against which microbes are antibiotics used?

(a) Fungi

(b) Virus

(c) Bacteria

(d) All of the above

Correct choice: (c)

Want more help with your biology homework?

We've got you covered with step-by-step solutions to millions of textbook problems, subject matter experts on standby 24/7 when you're stumped, and more.
Check out a sample biology Q&A solution here!

*Response times may vary by subject and question complexity. Median response time is 34 minutes for paid subscribers and may be longer for promotional offers.

Search. Solve. Succeed!

Study smarter access to millions of step-by step textbook solutions, our Q&A library, and AI powered Math Solver. Plus, you get 30 questions to ask an expert each month.

Tagged in

Marine biology

Marine ecology


Microbes Homework Questions from Fellow Students

Browse our recently answered Microbes homework questions.

Search. Solve. Succeed!

Study smarter access to millions of step-by step textbook solutions, our Q&A library, and AI powered Math Solver. Plus, you get 30 questions to ask an expert each month.

Tagged in

Marine biology

Marine ecology