Axenic

In biology, axenic describes a culture of a particular 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. Axenic cultures are useful because all of the organisms present within them are identical or share the same gene pool. Consequently they will generally respond in a more uniform and reproducible fashion, simplifying the interpretation of experiments.

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 only those cultures consisting of the desired type of organism produces the axenic culture. Subcultures that originate from a single organism or cell without genetic change are considered clones.

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.