Luria-Delbruck experiment
Encyclopedia
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, genetic mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
s arise in the absence of selection
Selection
In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...
, rather than being a response to selection. Therefore, Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
's theory of natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
acting on random mutations applies to bacteria as well as to more complex organisms. Max Delbrück
Max Delbrück
Max Ludwig Henning Delbrück was a German-American biophysicist and Nobel laureate.-Biography:Delbrück was born in Berlin, German Empire...
and Salvador Luria won the 1969 Nobel Prize in Physiology or Medicine
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...
in part for this work.
In their experiment, Luria and Delbrück inoculated a small number of bacteria into separate culture
Microbiological culture
A microbiological culture, or microbial culture, is a method of multiplying microbial organisms by letting them reproduce in predetermined culture media under controlled laboratory conditions. Microbial cultures are used to determine the type of organism, its abundance in the sample being tested,...
tubes. After a period of growth, they plated equal volumes of these separate cultures onto agar
Agar
Agar or agar-agar is a gelatinous substance derived from a polysaccharide that accumulates in the cell walls of agarophyte red algae. Throughout history into modern times, agar has been chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium...
containing phage
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...
(virus). If virus resistance in bacteria were caused by a spontaneous activation in bacteria—i.e., if resistance were not due to heritable genetic components, then each plate should contain roughly the same number of resistant colonies. This, however was not what Delbrück and Luria found. Instead, the number of resistant colonies on each plate varied drastically.
Luria and Delbrück proposed that these results could be explained by the occurrence of a constant rate of random mutations in each generation of bacteria growing in the initial culture tubes. Based on these assumptions Delbrück derived a probability distribution (now called the Luria–Delbrück distribution) that gives a relationship between moments consistent with the experimentally obtained values. The distribution that follows from the directed adaptation hypothesis (a Poisson distribution
Poisson distribution
In probability theory and statistics, the Poisson distribution is a discrete probability distribution that expresses the probability of a given number of events occurring in a fixed interval of time and/or space if these events occur with a known average rate and independently of the time since...
) predicted moments inconsistent with the data. Therefore, the conclusion was that mutations in bacteria, as in other organisms, are random rather than directed.
The results of Luria and Delbrück were confirmed in more graphical, but less quantitative, way by Newcombe. Newcombe incubated bacteria in a Petri dish
Petri dish
A Petri dish is a shallow glass or plastic cylindrical lidded dish that biologists use to culture cells or small moss plants. It was named after German bacteriologist Julius Richard Petri, who invented it when working as an assistant to Robert Koch...
for a few hours, then replica plated
Replica plating
In molecular biology and microbiology, replica plating is a technique in which one or more secondary Petri plates containing different solid selective growth media are inoculated with the same colonies of microorganisms from a primary plate , reproducing the original spatial...
it onto two new Petri dishes treated with phage. The first plate was left unspread, and the second plate was then respread, that is, bacterial cells were moved around allowing single cells in some colony to form their own new colonies. If colonies contained resistant bacterial cells before entering into contact with the phage virus, one would expect that some of these cells would form new resistant colonies on the respread dish and so to find a higher number of surviving bacteria there. When both plates were incubated for growth, there were actually as much as 50 times greater number of bacterial colonies on the respread dish. This showed that bacterial mutations to virus resistance had randomly occurred during the first incubation. Once again, the mutations occurred before selection was applied.
More recently, the results of Luria and Delbrück were questioned by Cairns and others, who studied mutations in sugar metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
as a form of environmental stress. However, eventually this result was found to have been caused by selection for gene amplification and/or a higher mutation rate
Mutation rate
In genetics, the mutation rate is the chance of a mutation occurring in an organism or gene in each generation...
in cells unable to divide. Nevertheless, there is no evidence yet for directed mutagenesis: only those mutations that allow the cells to respond to the environmental stress accumulate in a growing population.