Axenic

In biology, axenic describes a culture of an organism that is entirely free of all other "contaminating" organisms. The earliest axenic cultures were of bacteria or unicellular eukaryotes, but axenic cultures of many multicellular organisms are also possible.^[1]

Preparation

Axenic cultures of microorganisms are typically prepared using a dilution series of an existing mixed culture. This culture is successively diluted to the point where subsamples of it contain only a few individual organisms, ideally only a single individual (in the case of an asexual species). These subcultures are allowed to grow until the identity of their constituent organisms can be ascertained. Selection of those cultures consisting solely of the desired organism produces the axenic culture.

Axenic cultures are usually checked routinely to ensure that they remain axenic. One standard approach with microorganisms is to spread a sample of the culture onto an agar plate, and to incubate this for a fixed period of time. The agar should be an enriched medium that will support the growth of common "contaminating" organisms. Such "contaminating" organisms will grow on the plate during this period, identifying cultures that are no longer axenic.

Experimental use

As axenic cultures are derived from very few organisms, or even a single individual, they are useful because the organisms present within them share a relatively narrow gene pool. In the case of an asexual species derived from a single individual, the resulting culture should consist of identical organisms (though processes such as mutation and horizontal gene transfer may introduce a degree of variability). Consequently, they will generally respond in a more uniform and reproducible fashion, simplifying the interpretation of experiments.

Problems

The axenic culture of some pathogens is complicated because they normally thrive within host tissues which exhibit properties that are difficult to replicate in vitro. This is especially true in the case of intracellular pathogens. However, careful replication of key features of the host environment can resolve these difficulties (e.g. host metabolites, dissolved oxygen), such as with the Q fever pathogen, Coxiella burnetii.^[2] Most bacterial pathogens can be cultured more easily.^{[citation needed]}

References

^ Thain, M.; Hickman, M. (1994), Dictionary of Biology (9th edition), Penguin Books, London, UK, ISBN 0-14-051288-8
^ Omsland, Anders (2009). "Host cell-free growth of the Q fever bacterium Coxiella burnetii". PNAS. 106: 4430–4434. doi:10.1073/pnas.0812074106. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

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[1] Thain, M.; Hickman, M. (1994), Dictionary of Biology (9th edition), Penguin Books, London, UK, ISBN 0-14-051288-8

[2] Omsland, Anders (2009). "Host cell-free growth of the Q fever bacterium Coxiella burnetii". PNAS. 106: 4430–4434. doi:10.1073/pnas.0812074106. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

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