Skip main navigation

What are microbes?

A microbe is a micro-organism with microscopic or sub-microscopic size, meaning it cannot be seen by the naked eye.
Person in laboratory looking at a sample through a microscope

Here, we will define what microbes are and look at the different types of microbes. A microbe is a micro-organism with microscopic or sub-microscopic size, meaning it cannot be seen by the naked eye.

What are the different types of microbe?

Viruses are dependent on cellular synthesis machinery to replicate. This can often, but not always, result in cell death. Examples of viruses include measles, influenza, HIV, and Zika.

Bacteria are prokaryotes, meaning they lack a nucleus or other organelles. Most, but not all, are free-living. Examples of bacteria include tuberculosis, skin and wound infections pneumonia, and diarrheal diseases.

Fungi are eukaryotes, meaning they have nuclei and organelles. Fungi can exist as yeasts or moulds: yeast are single-celled organisms (e.g. Saccharomyces cerevisiae), while moulds form tubular extensions known as hyphae (fuzzy structures seen on old bread). Most fungi are larger than bacteria, and they have a rigid external cell wall and generally grow more slowly than bacteria. Examples of fungi include vaginal and oral candidiasis, fungal meningitis, aspergillosis, athlete’s foot, and toenail fungus.

Protozoa are single-celled eukaryotes and can be differentiated from algae because they are unable to carry out photosynthesis. Examples of protozoa include malaria, leishmaniasis, and giardiasis.

Prions are misfolded proteins with the ability to transmit their misfolded shape onto normal variants of the same protein.

Size comparison of microbes

Virus Bacteria Fungi/Protozoa
0.03-0.3 μm 0.1 – 10μm 4 – 10μm
Electron microscope Light microscope Visible to naked eye


This article is from the free online

Introduction to Practical Microbiology

Created by
FutureLearn - Learning For Life

Reach your personal and professional goals

Unlock access to hundreds of expert online courses and degrees from top universities and educators to gain accredited qualifications and professional CV-building certificates.

Join over 18 million learners to launch, switch or build upon your career, all at your own pace, across a wide range of topic areas.

Start Learning now