 Pure culture

A pure culture is a growing population intended to contain one microbial species or strain, so that observed properties can be attributed to that organism rather than an unresolved mixture. In bacterial culture, obtaining such a population makes physiological measurements and identification interpretable.^[ref]

The underlying experimental idea is spatial separation. On a solid culture medium, a separated colony provides material from a distinct founding unit. An isolated colony can supply a candidate pure culture; purity still needs an appropriate check, because appearance alone need not establish identity or exclude an unseen mixture.^[ref]

Consider a constructed growth measurement. If a mixed culture contains 100 cells of organism A and 10 of organism B, and their numbers increase by factors of 2 and 20 over the observation interval, the final counts are 200 and 200. Total growth from 110 to 400 cells does not reveal which organism grew fastest. Studying the separated populations permits the species-specific question to be tested.

Purity also differs from universal genetic identity: descendants can acquire mutations, and a culture's observable phenotype depends on conditions. The scientific objective is a defensible connection between the tested organism and the measured behaviour, with the intended population and detection criterion stated.