Luria–Delbrück experiment

The Luria - Delbrück experiment (also fluctuation test) is an imaginary and arranged by Salvador Edward Luria and Max Delbrück experiment, which in 1943 was published and that was a part of the work, in 1969 together with Alfred Hershey with the Nobel Prize for the Luria and Delbrück Physiology or Medicine were awarded.

The experiment in which the development of resistance in bacteria was investigated against bacteriophages showed that random mutations in bacteria and thus happen spontaneously even in the absence of selection mechanisms, and refuted the alternative hypothesis that this could happen in response to changing conditions in the environment. Using a mathematical model developed by Delbrück, the hypotheses were tested in a quantitative manner.

In addition, Luria and Delbrück were able to determine with the help of some model assumptions, the mutation rate of the bacteria. To this end, fluctuation tests are still being carried out.

Initial situation

It was the time of the experiment has long been known that a bacterial culture is exposed to a deadly bacteriophage is first decimated by this. After some time, the seemingly eliminated culture grows again, because a resistant variant of the bacterium has been formed that can grow unhindered. Unknown, however, the early 1940s was the mechanism of mutations, especially if they are done before adding the virus or as a reaction to this. Also about the molecular basis of heredity, little was known, DNA as the carrier of genetic information was not yet discovered.

At this time, led Luria experiments on bacteriophages by. One of his problems was here that the number was subjected to phage resistant bacteria that have become very large fluctuations. The idea for the experiment came Luria when viewing a colleague at a slot machine. The machine kept usually a use of the player and threw it in rare cases, the entire jackpot from. Luria realized that the fluctuations were not seen to be too insurmountable barrier, but is itself the object of analysis should be made. The trained physicist Max Delbrück, the Luria reported about his idea worked from thereon the theoretical foundations of the experiment.

Description of the experiment

In the experiment, a larger number of separate cultures of the bacterium Escherichia coli was formed. Initially, each 50 to 100 non-resistant bacteria increased in a nutrient solution until colonies originated from about 109 bacteria. Then, each of the cultures was exposed to a high concentration of bacteriophages nutrient plates of the lethal for non-resistant bacteria type T1. After a waiting period of 24 and 48 hours the number of colonies ( more resistant ) bacteria were determined and finally determined from the data of many samples, the resulting distribution in each case.

The main aim of the experiment was to distinguish between two alternative hypotheses concerning the mutation: According to the first hypothesis, the development of resistance in bacteria is in response to the attack of the phage. With a low probability of individual bacteria can independently develop a resistance to the phage, which they hand down to their descendants. In this case corresponds to the expected distribution of the cultures to a Poisson distribution. The fluctuations are relatively small, since the variance of a Poisson distribution is equal to its average value. If this hypothesis of " acquired immunity " true, would see the results as in the left column of the chart.

According to the second hypothesis -forming resistance mutations happen constantly with a small probability, so even before the bacterial culture was attacked by the phage. In most cultures, these mutations occur late, when the initially 50 to 100 bacteria have already proliferated. The resistant bacteria have little time to multiply, and at the time of the bacteriophage attack only a few resistant bacteria are present, resulting in low measured concentrations. In a few cultures, however, happen early resistance forming mutations, so that surviving after prolonged exponential growth, a large number of bacteria attack the phage ( see right-hand column of the chart ). In this way, the measured distribution of a variance that is greater by a multiple than its average value. There are rare events ( "Jackpot " ) in which an extremely large number of bacteria is measured. A mathematical model developed by Delbrück allowed to specify a formula for the variance of the expected distribution and to determine the mutation rate approximation.

The experiment clearly confirmed the validity of the second hypothesis. The measured variance of the distribution was many times greater than the average, and was in good agreement with the model predictions. Thus, the existence of permanent random mutations in bacteria was shown.

Aftereffect

Often, the fluctuation test was interpreted to mean that he show the validity of the Darwinian theory of evolution in bacteria and Lamarckian theories disprove. Luria himself had described the bacteriology as " the last bastion of Lamarckism ".

Luria and Delbruck himself, however, were initially cautious when it comes to Verallgemeinbarkeit their results. Their results were confirmed in the following years by further experiments in which, for example, instead of resistance to bacteriophages, resistance to penicillin or X-rays was tested.

While Luria and Delbrück calculated only the mean and variance of the expected according to the theory of spontaneous mutations from their distribution model, succeeded Lea and Coulson 1949, derive a mathematical expression for the distribution itself. It is called the Luria - Delbrück distribution and is still used today for the determination of mutation rates.

In the late 1980s the generality of the randomness of mutations by a work of John Cairns and employees has been questioned. Cairns examined in contrast to Luria and Delbrück cases of non-lethal selection (ie, the bacteria can survive under selection pressure, but do not proliferate ) and found that for the bacteria favorable mutations occurred significantly more often than could be explained by random mutations. After a long controversial scientific debate about this, known as Adaptive mutation phenomenon is now well known that different mechanisms exist that can increase response to an environmental selection pressure, the frequency of mutations by the organisms, for example, by the formation of error-prone polymerases. However, the existence of targeted mutations in response to external selection pressure could not be shown.