What is a Microbe? Why “Microbe” Is Difficult to Define

What is a microbe? On the surface, it seems easy to define. Many would say a microbe is an organism that is microscopic, one that is invisible to the naked eye. Many also include bacteria, archaea, fungi, viruses, protists, and even prions in the realm of microbiology. But, there are numerous nuances that challenge the “must be microscopic” definition of a microbe. Not all fungi are microscopic, it’s debated whether viruses are even living organisms, and prions are misfolded proteins.

Below are some examples of the inconsistencies that make the term “microbe” hard to define.

Bacteria Visible to the Naked Eye

While most microbes are invisible to the naked eye, some microbes can just barely be seen without a microscope. Epulopiscium fishelsoni, for example, is about the size of a grain or salt. This bacterium, identified in 1985, has an internal volume of more than 2,000 times that of a typical bacterium and ranges from 200 – 700 microns in length. 

Epulopiscium fishelsoni is not the only “giant microbe.” Thiomargarita namibiensis, found in ocean sediments in 1997, is typically  0.1–0.3 mm in length, but can be up to 0.75 mm in size.  An even bigger bacterium of the same genus, Thiomargarita magnifica, was found in Caribbean mangroves. This microbe, if you could call it that, is up to two centimeters long. And unlike other bacteria where genetic material runs loose within the cell, T. magnifica contains thousands of membrane-bound structures that store genetic information of the cell. This finding goes against one of the defining characteristics of bacteria and makes this microbe seem almost eukaryotic.

Sometimes Fungi are Called Microbes, But What About Mushrooms?

Speaking of eukaryotes, let’s take a look at fungi. Yeasts and molds, which are unicellular, and mushrooms, which are multicellular, are all considered fungi. If fungi are considered microbes, does this make mushrooms microbes?

Mushrooms do not meet the “must be microscopic” definition of a microbe. Mushrooms are also considered multicellular organisms with differentiated structures making them difficult for us to think of as microbes (though some bacteria exhibit multicellularity). Mushrooms have mycelium, which is the threadlike structure found in the soil, and fruiting bodies, which are what we know as “mushrooms” and produce spores. 

Nonliving Infectious Agents – Viruses and Prions

This next section takes us away from the discussion of living things to a discussion of what some consider as nonliving infectious particles: viruses and prions.

While there are different definitions of living organisms (the question what is life, is a whole other topic up for debate), many scientists use all or a subset of the following as the criteria for life: (1) composed of cell(s), (2) have a life cycle / can reproduce, (3) undergo metabolism, (4) can maintain homeostasis, (5) can respond to stimuli, and (6) can evolve.

Viruses have some of these characteristics of life but not all and are often thought of existing at the “edge of life.” Viruses do evolve, but they aren’t made of cells and require a host cell to reproduce to produce new viral particles. However, obligate intracellular bacteria such as Chlamydia trachomatis and Rickettsia spp. also require a host cell to replicate. But with their “bacterial” status, these intracellular bacteria are typically considered microbes. Another example is Carsonella ruddii, which has been called an obligate endosymbiont bacterium. However, after examining its genome, researchers suspect C. ruddii straddles the line between a living cell and an organelle.

To complicate things further, let’s look at prions. Prions are a type of protein that can cause other proteins to fold abnormally to cause disease. Although prions cause disease, they are unlike bacteria – and even viruses – since they don’t contain any genetic material.

TL;DR

Endosymbionts, viruses, prions, fungi, and large bacteria all demonstrate that there are no discrete characteristics of microbes separate from other forms of life. Microbes blur the lines between the microscopic and the macroscopic, living and nonliving, and sometimes sit at the edge of prokaryote and eukaryote. It’s difficult to come up with a distinct definition of a microbe as there are always exceptions to the norm and reminds us that evolution is a continual process where organisms such as microbes might be difficult to categorize.

Further Reading

Are Viruses Alive? Scientific American. 2008.

I loved reading this discussion about the living vs. nonliving status of viruses and what that means for virology research.

Bacterial Ventures into Multicellularity: Collectivism through Individuality. PLoS One. 2015.

A primer about bacterial multicellularity.

Featured image by Christina Liu for IGI.