Microorganisms and their position in the biological hierarchy

Commercial microbiology uses a wide range of microorganisms, including bacteria, microalgae and fungi. However, microorganisms are just one - but vitally important - part of the biological world.

This section of the website will take a look at where microorganisms fit in with other organisms and will then lead into a more detailed description of the various microorganisms themselves.
Scientists have developed a number of systems for the classification of organisms. As more evidence is accumulated, and new technologies are developed, these systems change. There are strong proponents of each variation in the classification system.

One system currently used is the Three Domain System. In this system organisms are primarily classified according to differences in their ribosomal RNA (rRNA). Ribosomes are complex molecules found in cells that are made up of rRNA and protein. They are the mechanism by which cells produce proteins.

The three Domains are:
  • Archaea
  • Bacteria
  • Eukarya

Each Domain consists of one or more Kingdoms:
  • Archaea - Kingdom Archaebacteria
  • Bacteria - Kingdom Eubacteria
  • Eukarya - Kingdom - Protista
  • Eukarya - Kingdom - Fungi
  • Eukarya - Kingdom - Plantae
  • Eukarya - Kingdom - Animalia

Each Kingdom is divided into one or more of Phyla (singular Phylum), each Phylum is divided into one or more Classes, each Class is divided into one or more Orders, each Order is divided into one or more Families, each Family is divided in to one or more Genera (singular Genus), and each Genus is divided into one more Species. Each Species can then consist of a number of strains.

A mnemonic to help in remembering this classification system is Do Keep Plates Clean OFamily Gets Sick.

Commercial microorganisms are found in a number of these Kingdoms so we will take a quick look at each of them before going into more detail.

Kingdom - Archaebacteria
Kingdom - Eubacteria
Kingdom - Protista
Kingdom - Fungi

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