Oxygen is present as a major constituent (20 per cent) of our atmosphere, and most life forms are dependent upon it for survival and growth. Such organisms are termed aerobes. Not all organisms are aerobes however; some anaerobes are able to survive in the absence of oxygen, and for some this is actually a necessity.

Aerobic organisms require oxygen to act as a terminal electron acceptor in their respiratory chains (see Chapter 6). Such organisms, when grown in laboratory culture, must therefore be provided with enough oxygen to satisfy their requirements. For a shallow layer of medium such as that in a petri dish, sufficient oxygen is available dissolved in surface moisture. In a deeper culture such as a flask of broth however, aerobes will only grow in the surface layers unless additional oxygen is provided (oxygen is poorly soluble in water). This is usually done by shaking or mechanical stirring.

Obligate anaerobes cannot tolerate oxygen at all (see Box 5.4). They are cultured in special anaerobic chambers, and oxygen excluded from all liquid and solid media. Facultative anaerobes are able to act like aerobes in the presence of oxygen, but have the added facility of being able to survive when conditions become anaerobic. Aerotolerant anaerobes are organisms that are basically anaerobic; although they are not inhibited by atmospheric oxygen, they do not utilise it. Microaerophiles require oxygen, but are only able to tolerate low concentrations of it (2-10 per cent), finding higher concentrations harmful. Organisms inoculated into a static culture medium will grow at positions that reflect their oxygen preferences (Figure 5.7).

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