S. E. Luria and M. Delbruck (1943). Mutations of bacteria from vrius sensitivity to virus resistance.
Genetics 28:491
[Presented by: Steve Carr (scarr@mun.ca), 14 January 2014]
Background
Max Delbruck (1906 -1981) & Salvador Luria (1912 -1991)
Shared 1969 Nobel Prize in Physiology or Medicine
Bacteriology in 1940s not heavily influenced by genetic thinking
Bacteria have no nuclei: do they have "genes"?
Bacterial "phenotypes" are the manifestations of 106s of bacteria simultaneously
Bacteria don't have sex: crosses not possible
[Discovery of bacterial sex led to 1958 Nobel Prize]
bacteriophages ("phages") - "subcellular parasites that infect, multiply within, & kill bacteria."
T1 phages are active on E. coli
[phage] >> [bacteria] no bacterial colonies grow: bacteria are Tons ("T-one sensitive")
[phage] ~ [bacteria] some bacterial colonies grow: bacteria are Tonr ("T-one resistant")
Tonr phenotype is stable
all descendant bacteria are Tonr
phenotype persists in the absence of T1
Two Hypotheses (d'Herelle 1926 vs Brunet 1929)
1. Tonr phenotype is induced by exposure of bacteria to phage
Each bacterium has a (small, finite) chance of survival (~ 1 / 107);
Survivors have altered metabolic phenotype, which is transmitted to offspring
[distinction between phenotype & genotype not clear]
Bacteria adapt to their environment :
a Lamarckian hypothesis: inheritance of acquired characteristic
2. Tonr phenotype occurs spontaneously, prior to exposure of bacteria to phage
Some rare bacteria (say ~ 1 / 107) are already Tonr
These bacteria have undergone genetic mutation to a stable genotype
[phenotype persists in absence of phage]
a Darwinian hypothesis: Tonr bacteria are selected
Materials & Methods
Hypotheses make different predictions as to
numerical distribution of Tonr phenotypes among bacterial cultures.
Induction (Adaptation) Hypothesis predicts: n / N = a
where n = number of Tonr bacteria observed out of
N = number of Tons bacteria plated, and
a = probability of conversion from Tons to Tonr
Then, n should be a constant fraction of N
Mutation Hypothesis predicts: n / N = ga2g / 2g = ga
where a = mutation rate (# mutations / cell / generation)
g = # generations to go from 1 N bacteria, so that
N = 2g doublings occur, of which
n = ga2g produce mutant Tonr bacteria
