What Is a Microbe?
Microbes, also called “microorganisms,” are tiny living things that cannot be seen with the naked eye. Most microbes are single-cell organisms, while others are made up of only a few cells.
Even though they have relatively simple bodies, microbes can perform an extraordinary range of functions to survive. Many can grow in the absence of oxygen, and many can extract nutrients from complex chemicals, such as plastics and detergents, that larger organisms can’t eat.
Where are microbes found?
Microbes can be found anywhere on Earth where life is known to exist. This includes environments familiar to humans as well as extreme environments, such as arsenic-contaminated lakes, Antarctic lakes, and hydrothermal vents on the ocean floor that reach 340 degrees Celsius (700 degrees Fahrenheit).
Our bodies—and those of other living things—also contain a large number of microbes that help with processes such as digestion. (The collection of microbes in our digestive systems is commonly referred to as our gut microbiome.) Other microbes in our bodies are associated with infection or disease.
How many kinds of microbes are there?
The term "microbe" groups organisms together based only on their size. So, unlike more technical terms, it encompasses a broad array of biological diversity.
Genetically, microbes are represented in all three domains of life: bacteria, archaea, and eukarya, the last of which includes all plants and animals.
Microbes can be divided into different categories based on genetics and the structure of their cells. Microbes in each category differ in the composition of their outer walls. They differ in how they reproduce. They also differ in some of the processes they use to carry out cellular activities.
The different kinds of microbes can also be separated based on whether or not they have a cell nucleus, a membrane-bound compartment inside the cell that stores genetic material. Cells that have a nucleus are called eukaryotes. Cells that do not have this special compartment to store their genetic material are called prokaryotes. Eukaryotes also have other membrane-bound compartments that prokaryotes typically do not have.
- Bacteria: Bacteria are single-cell, prokaryotic organisms. They usually lack internal membrane-bound structures, though there are some exceptions. Bacteria reproduce by binary fission, in which one cell copies its genetic material and divides into two without sex. They have cell walls that are composed of peptidoglycan, a complex of protein and sugars. Some have an additional capsule outside of their cell wall.
- Archaea: Archaea resemble bacteria; they are single-cell prokaryotic organisms that reproduce through binary fission, and they have cell walls, though the walls are composed of sugars (polysaccharides). But the two differ in their cellular machinery: The way archaea read their genes and make proteins is genetically more similar to eukaryotes than to bacteria. Many archaea are found in extreme environments.
- Fungi: Fungi can be made up of one cell or many, and larger fungi, such as mushrooms, are not considered microbes. Single-celled fungi include yeasts. Fungi are eukaryotic cells that typically live stationary lives. They can reproduce sexually or asexually, and often make use of resilient spores, which act like the seeds of a plant. Fungi can have cell walls that are made of chitin, a polysaccharide compound also found in insects.
- Protists: The name "protist" is used to describe eukaryotes that don't fall into the categories of fungi, plant, or animals. They are typically single-cell organisms and include amoeba, algae, dinoflagellates, and euglena. Protists take on many different shapes and sizes. They can typically reproduce through binary fission.
- Viruses: Viruses are tiny infectious agents that occupy a gray area between the living and nonliving realms. Although they are not technically alive, they are generally included in the category of "microbes," especially when referring to infectious agents. Viruses cannot grow and reproduce on their own, making them parasites that require a host cell. They have a relatively simple structure composed of genetic material surrounded by either a protein shell, called a capsid, or by a membrane, called an envelope. This envelope is made of lipids (fat or fat-like substances that can be broken down by soap and other detergents) derived from the host cell and proteins.
