卢瑞亚-德尔布吕克实验

分子生物学

对噬菌体T1的抗性机制似乎是由于fhuA基因的突变所致——fhuA是一种膜蛋白，作为T1的受体。 tonB基因产物也是T1感染所必需的。FhuA蛋白积极参与Ferrichrome、 阿波霉素和利福霉素的转运。 它还赋予细菌对微菌素J25（Microcin J25）和肠毒素M（colicin M）的敏感性，并作为噬菌体T5、phi80以及T1的受体。

The Luria–Delbrück experiment (1943) (also called the Fluctuation Test) demonstrated that in bacteria, genetic mutations arise in the absence of selective pressure rather than being a response to it. Thus, it concluded Darwin's theory of natural selection acting on random mutations applies to bacteria as well as to more complex organisms. Max Delbrück and Salvador Luria won the 1969 Nobel Prize in Physiology or Medicine in part for this work.

Simple model

Suppose a single bacterium is introduced to a growth medium with rich nutrients, and allowed to grow for N× of its doubling time, we would obtain 2^(N) offsprings. Then, we introduce a challenge by bacteriophages. This would kill off most bacteria, but leave some alive. We can then smear the growth medium over a new growth medium, and count the number of colonies as the number of survivors.

In the Lamarckian scenario, each bacterium faces the challenge alone. Most would perish, but a few would survive the ordeal and found a new colony. In the Darwinian scenario, resistance to the phage would randomly occur during the replication. Those that inherited the resistance would survive, while those that did not would die.

History

By the 1940s the ideas of inheritance and mutation were generally accepted, though the role of DNA as the hereditary material had not yet been established. It was thought that bacteria were somehow different and could develop heritable genetic mutations depending on the circumstances they found themselves: in short, was the mutation in bacteria pre-adaptive (pre-existent) or post-adaptive (directed adaption)?

In their experiment, Luria and Delbrück inoculated a small number of bacteria (Escherichia coli) into separate culture tubes. After a period of growth, they plated equal volumes of these separate cultures onto agar containing the T1 phage (virus). If resistance to the virus in bacteria were caused by an induced activation in bacteria i.e.

Description of the test

A small number of cells are used to inoculate parallel cultures in a non-selective medium. The cultures are grown to saturation to obtain equal cell densities. The cells are plated onto selective media to obtain the number of mutants (r). Dilutions are plated onto rich medium to calculate the total number of viable cells ( Nt). The number of mutants that appear in the saturated culture is a measure of both the mutation rate and when the mutants arise during the growth of the culture: mutants appearing early in the growth of the culture will propagate many more mutants than those that arise later during growth. These factors cause the frequency (r/Nt) to vary greatly, even if the number of mutational events (m) is the same.

Distribution

In all these models the mutation rate (μ) and growth rate (β) were assumed to be constant. The model can be easily generalized to relax these and other constraints. These rates are likely to differ in non experimental settings. The models also require that Nt μ >> 1 where Nt is the total number of organisms. This assumption is likely to hold in most realistic or experimental settings.

Luria and Delbrück estimated the mutation rate (mutations per bacterium per unit time) from the equation （公式略）

where β is the cellular growth rate, n0 is the initial number of bacteria in each culture, t is the time, and （公式略）

where Ns is the number of cultures without resistant bacteria and N is the total number of cultures.

Molecular biology

The mechanism of resistance to the phage T1 appears to have been due to mutations in the fhuA gene - a membrane protein that acts as the T1 receptor. The tonB gene product is also required for infection by T1. The FhuA protein is actively involved in the transport of ferrichrome, albomycin and rifamycin. It also confers sensitivity to microcin J25 and colicin M and acts as a receptor for the phages T5 and phi80 as well as T1.

The FhuA protein has a beta-barrel domain (residues 161 to 714) that is closed by a globular cork domain (residues 1 to 160). Within the cork domain is the TonB binding region (residues 7 to 11).