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Micro-organisms can come from the three domains of life - archaea, bacteria and some species of eukaryote. They are able to metabolise a wide variety of substrates. This allows micro-organisms to survive in a wide range of ecological niches.

Micro-organisms are robust enough to be used by humans in research and industry. Use of micro-organisms has the following benefits:

• they are adaptable
• They can be easily cultivated
• They have fast rates of growth
• They produce a wide range of products from their metabolic pathways which can be used for human benefit

In order to grow, micro-organisms require a source of energy and simple chemical compounds (raw materials). These will be used in order to synthesise other more complex molecules necessary for life (this is called biosynthesis). The energy source can either be from light in photosynthesis or chemical substrates.

Fermenters are pieces of apparatus that maintain optimal conditions for the growth of micro-organisms.

Although some micro-organismscan produce all the complex molecules required for biosynthesis (e.g. amino acids, vitamins and fatty acids), some need these complex molecules to be added artificially in a growth medium.