Fluctuation test

Fluctuation test
(Science: investigation) test devised by Luria and Delbruck to determine whether genetic variation in a bacterial population arises spontaneously or adaptively. in the original version the statistical variance in the number of bacteriophage resistant cells in separate cultures of bacteriophage sensitive cells was compared with variance in replicate samples from bulk culture. The greater variance in the isolated populations indicates that mutation occurs spontaneously before challenge with phage. (The proportion of resistant cells depends upon when after isolation the mutation arises which will be very different in separate populations).
The fluctuation test shows that mutations occur randomly in a growing culture, and are not induced by the selection agent. The fluctuation test shows that mutations occur randomly in a growing culture, and are not induced by the selection agent. If mutations were induced, for example by streptomycin on the plates, all the plates would have approximately the same number of colonies. In contrast, if mutations occured randomly in the growing population, when the culture was subdivided some tubes contained a population of cells in which the mutation occurred early in the growth period, whereas in others it may have occurred late, or not at all. There will be many more colonies on the plate if the mutation arouse early during growth than if the mutation arouse late in the growth period because the mutants would have undergone more rounds of replication in the former case.If mutations were induced, for example by streptomycin on the plates, all the plates would have approximately the same number of colonies. In contrast, if mutations occured randomly in the growing population, when the culture was subdivided some tubes contained a population of cells in which the mutation occurred early in the growth period, whereas in others it may have occurred late, or not at all. There will be many more colonies on the plate if the mutation arouse early during growth than if the mutation arouse late in the growth period because the mutants would have undergone more rounds of replication in the former case.