Luria: Mutations of Bacteria and of Bacteriophage

"It was in January 1943, after I had moved to Indiana University in Bloomington, that I thought of a critical choice between spontaneous and phage-induced resistance. The first written statement of it I find in a letter to Delbruck dated January 20: 'I thought that a clean cut experiment would be to find out how the fluctuations in the number of phage-resistants depend on the culture from which they come. That is: If I plate with phage ten samples of the same culture of E. coli, I find numbers of resistants which fluctuate according to Poisson's law. If I plate 10 samples of 10 different cultures of E. coli, all containing the same amount of E. coli, I find much larger fluctuations. If the resistants were reproduced on the plate, after contact with phage, they should show the same fluctuations in both cases.' The idea of this experiment came to me, in fact, while watching the fluctuating returns obtained by various colleagues of mine gambling on a slot-machine at the Bloomington Country Club, where faculty dances were then held one Saturday a month."

MGA2-10-27

NB: Luria from Italy and Delbruck from Germany were both interned at Indiana U as "enemy aliens", even though both were of Jewish ancestry. At Bloomington, they were both subject to strict curfews. "Fluctuations" here has the technical meaning of how individual observation differ from the expectation of all observations. In biology and elsewhere, "fluctuations" are measured as the statistical variance, the average of the squared values of the individual deviations from the mean of the Normal distribution. In this case, fluctuations are measured as departures from the Poisson distribution, a special case of the normal distribution when events are rare.