"Microbe" is short for "microorganism". Microbes are therefore microscopically small organisms - living beings that cannot be seen with the naked eye. This very broad definition is based on a single criterion: the resolving power of the human eye. According to this definition, microbes are all life forms with a diameter of less than about 0.1 millimetres. In practice, however, this definition is restricted to a greater or lesser extent for various reasons.

Viruses are usually excluded because they are not independent living organisms. In addition, unlike all other microbes, they remain invisible even under a light microscope. Only an electron microscope can reveal their nanometre-sized structures. However, there are exceptions, such as the mimivirus, which is the size of a small bacterium.

There is a consensus that all prokaryotes, i.e. all bacteria and archaea, are microbes. They are unicellular, but can form chains or other cell clusters. In contrast to the cells of eukaryotes, they do not have a nucleus, i.e. their DNA is not enclosed in a double-walled envelope. They also lack other typical cell components of eukaryotes, such as mitochondria and chloroplasts. These organelles were originally independent microbes before they entered into a symbiosis and lost their independence inside other microbes. Many prokaryotes are about one micrometre thick and some can be several micrometres long. One exception is the sulphur bacterium Thiomargarita namibiensis, which, with a diameter of up to 0.75 millimetres, is visible to the naked eye and would therefore, strictly speaking, no longer be a microbe.

Yeasts (such as Candida and Saccharomyces), unicellular algae (such as Chlorella), protozoa (such as amoebae) and slime moulds (such as Dictyostelium and Myxomycetes), which are distinguished from plants and animals from the point of view of evolutionary biology, also fulfil the criterion of small size. However, while yeasts are still generally categorised as microbes for historical reasons and because of similar laboratory techniques, microbiologists leave algae to botanists, protozoa to parasitologists and slime moulds to mycologists (although slime moulds are not fungi). This leaves us with the following definitions of microbes in the narrower sense: bacteria, archaea and yeasts. Admittedly, this is not logical or consistent.

Incidentally, in a very broad sense, humans and many other life forms also live for a time as "microorganisms" because they begin their existence as microscopically small fertilised egg cells.

© Text Joachim Czichos / VAAM, czichos[at]czience.de, Nutzung gemäß CC 4.0

Figure: Nanoarchaeum equitans is one of the smallest microbes in the world (400nm), (c) Karl O. Stetter, gemäß CC 4.0

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Was sind Mikroben? Video aus dem Life Science Learning Center, Zürich, Schweiz