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Modern Genetic Analysis Chapter 7. Gene Mutations The Molecular Basis of Mutation
Genetics In Process 7-1: Salvador Luria and Max Delbruck show that bacterial mutations are random

Bacteria and phages have played key roles in genetic research, including the study of mutation. One experiment by Luria and Delbruck in 1943, using bacteria and phages, was particularly influential in shaping our understanding of the nature of mutation not only in bacteria, but in organisms generally.It was known at the time that, if Escherichia coli bacteria are spread on a plate of nutrient medium in the presence of a phage called T1, the bacteria soon become parasitized and killed by the phages. However, rarely but regularly, colonies were seen that were resistant to phage attack; these colonies were stable and so appeared to be genuine mutants. However, it was not known if these mutants were produced spontaneously but randomly in time or if they were induced by the presence of the phage.

Luria reasoned that, if mutations occurred spontaneously, then the mutations might be expected to occur at different times in different cultures, so the resulting numbers of resistant colonies per culture should show high variation (or "fluctuation" in his words). He later claimed that he obtained the idea while watching the fluctuating returns obtained by colleagues gambling on a slot machine at a faculty dance in a local country club.

Luria and Delbruck designed their "fluctuation test" as follows. They inoculated 20 small cultures, each with a few cells, and incubated them until there were 108 cells per milliliter. At the same time, a much larger culture also was inoculated and incubated until there were 108 cells per milliliter. The 20 individual cultures and 20 aliquots of the same size from the large culture were plated in the presence of phage. The 20 individual cultures showed high variation in the number of resistant colonies: 11 plates had 0 resistant colonies, and the remainder had 1, 1, 3, 5, 5, 6, 35, 64, and 107 per plate. The 20 aliquots from the large culture showed much less variation from plate to plate, all in the range of 14 to 26. If the phage were inducing mutations, there was no reason why fluctuation should be higher on the individual cultures, because all were exposed to phage similarly. The best explanation was that mutation was occurring randomly in time: the early mutations gave the higher numbers of resistant cells because they had time to produce many resistant descendants. The later mutations produced fewer resistant cells. This result led to the reigning "paradigm" of mutation; that is, whether in viruses, bacteria, or eukaryotes, mutation can occur in any cell at any time.

The fluctuation test data also allowed calculation of the rate of mutations per cell division, and the approach is still used today in microbial genetics.

© 1999 by W. H. Freeman and Company. All rights reserved.