In biology, axenic (, ) describes the state of a culture in which only a single species, variety, or strain of organism is present and entirely free of all other contaminating organisms. The earliest axenic cultures were of bacteria or unicellular , but axenic cultures of many multicellular organisms are also possible.
Preparation
Axenic cultures of
are typically prepared by subculture of an existing mixed culture. This may involve use of a
serial dilution, in which a 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. Subculture selection may also involve manually sampling the target organism from an uncontaminated growth front in an otherwise mixed culture, and using this as an source for the subculture.
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
Cloning 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
Reproducibility fashion, simplifying the interpretation of
.
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
, dissolved oxygen), such as with the
Q fever pathogen,
Coxiella burnetii.
See also
-
Asepsis
-
Gnotobiotic animal
-
Sterilization (microbiology)
