Pure culture

In experimental biology, an axenic or pure culture is a population of cells or multicellular organisms growing in the absence of other species or types. A pure culture may originate from a single cell or single organism, in which case the cells are genetic clones of one another.

Methods of Isolation
Pure cultures of single-celled organisms usually must be isolated and grown under aseptic conditions, requiring sterilized instruments and filtered or still air. Isolated colonies of microorganisms are usually obtained by growth on the surface of a petri dish. The petri dish (or plate) contains an appropriate growth medium for the microorganism of interest, usually gelled with agar. To isolate a pure culture, the initial sample (inoculum) is manipulated using with an inoculation loop or needle to spread and dilute the cells on the surface of the plate. The objective is to eventually have some areas of the petri dish with isolated single cells. The culture is incubated under appropriate environmental conditions until the cells have grown and visible colonies appear. Well-isolated colonies have a high probability of having grown from single cells and therefore being pure cultures.

Pure cultures can also be prepared by high dilution from a liquid culture into a liquid medium. At sufficient dilution only a fraction of the inoculated culture tubes grow, and the probability is high that those cultures originated from a single cell.

Pure cultures of multicellular organsims are often more easily isolated by simplying picking out a single individual to initiate a culture. This is a useful technique for pure culture of fungi, multicellular algae, and small metazoa, for example.