A microorganism is a living organism that is too small to be seen without the aid of a microscope. Microorganisms are incredibly diverse and include bacteria and fungi as well as archae, protists, plankton and amoeba.Viruses are not considered to be living organisms but are often considered to be microorganisms because of their extremely small size.

The invention of the microscope allowed humans to look into the tiny world of microorganisms. Since their discovery, microorganisms have been found to inhabit almost every environment on earth including those thought previously to be uninhabitable, such as hot springs and acidic pools.

Microorganisms are essential to life on Earth

Humans had long used microorganisms to make bread, cheese, yoghurt wine and beer, but without knowing how these processes worked. We are also discovering that microorganisms are vital to the health of the planet by being an important part of nutrient cycling. Learn more about these essential services in the article Bacteria - good, bad and ugly.

We can now explain diseases as infections from microorganisms rather than spontaneously generated from ‘miasma’. Fortunately, we've learned how to control many infectious diseases.

Take up the challenge

The following activities help students deepen their understanding of microorganisms:

  • Exploring microorganisms asks students to research and summarise the key features of bacteria, viruses and fungi.
  • Grow your own fungus explores the conditions best suited to the growth of fungus using different types of food.
  • Make an adenovirus uses a template to make a model of an adenovirus and asks students to investigate the different shapes of different viral diseases and the similarities and differences between viral families.

Activity ideas

The Hub has numerous other activities that involve microorganisms.

    Published 9 November 2008, Updated 13 December 2016