Biology 1943

Mutations of Bacteria from Virus Sensitivity to Virus Resistance

Salvador Luria & Max Delbrück

The fluctuation test that proved bacterial mutations arise at random, before selection — not because of it.

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In depth · the introduction

Do bacteria turn resistant because a threat appears — or were the lucky few already mutants, waiting? Two scientists answered with arithmetic.

The big idea

Pour a virus onto a dish of bacteria and almost all of them die — but a few survive and breed. Where did those survivors' resistance come from? One idea: the virus itself prods a few cells into becoming resistant on the spot. The other: a handful of cells had already become resistant earlier, through random copying mistakes as they grew, and the virus simply spares them. Luria and Delbrück found a way to decide between the two — without ever watching a single mutation happen.

How it came about

The idea struck Salvador Luria in 1943 while he watched a colleague play a slot machine at a faculty dance: most plays pay nothing, but once in a while the machine hits a jackpot. If resistance came from random mutations during growth, bacterial cultures should behave like slot machines — most yielding only a few survivors, but the rare culture, where a mutation happened early and was then copied many times, hitting a jackpot of thousands. Luria wrote to the physicist-turned-biologist Max Delbrück, who supplied the mathematics. They grew many separate cultures, counted the survivors in each, and saw exactly that wild, jackpot-laden scatter.

Why it mattered

It proved that Darwin's logic reaches all the way down to bacteria: the variation comes first, blindly, and selection only picks from what is already there. It also turned 'how often do mutations happen?' into something you can actually measure, with counting and arithmetic — and it made bacteria, the workhorses of all later molecular biology, into legitimate subjects of genetics.

A way to picture it

Imagine handing out lottery tickets to a crowd that keeps growing over several days, then asking how many winners each room holds. If tickets were only handed out at the final door (immunity), every room ends up with about the same few winners. But if tickets were bought all along as the crowd multiplied (mutation), then a room where someone won early — and then brought along a huge family of fellow winners — will hit a jackpot, while most rooms have none. The jackpots betray that the winning happened before the door, not at it.

Where it sits

It is the moment bacteria join the story this Library tells about heredity. It stands beside Darwin's natural selection (1859) — showing the same blind-variation-then-selection at the microbial scale — and just ahead of Avery (1944) and Hershey–Chase (1952), which would reveal what the gene is actually made of. The phage research it launched runs straight on to the double helix and beyond.

The original document

Original source text

S. E. Luria & M. Delbrück · Indiana University & Vanderbilt University · Genetics 28 (1943): 491–511

The problem

When a bacterial culture is attacked by a bacteriophage, almost all the cells die, but a few resistant cells survive and found resistant colonies. Two explanations were on the table. By the acquired-immunity view, contact with the virus itself induces a small, fixed fraction of cells to become resistant. By the mutation view, rare resistant mutants already exist in the culture, having arisen by chance during earlier growth, independently of the virus.

The idea

Luria and Delbrück realised the two views make different statistical predictions, and that the difference shows up not in the average number of survivors but in how that number fluctuates from one culture to another. If resistance is induced at the moment of exposure, each culture is an independent series of rare events and the survivor counts should follow a Poisson distribution, with variance about equal to the mean. If resistance is inherited from a chance mutation during growth, a mutation that happened early is passed to a large clone of descendants, so an occasional culture carries a huge 'jackpot' of resistant cells and the counts fluctuate far more widely than Poisson allows.

The experiment

They grew many small parallel cultures of Escherichia coli from tiny inocula, let each grow undisturbed, then plated each entire culture on a lawn of bacteriophage and counted the resistant colonies. As a control, they sampled a single large culture many times over. The single culture, sampled repeatedly, gave counts that varied only by sampling (Poisson) error; the independent parallel cultures gave counts that fluctuated enormously, including rare jackpots.

The result

The wide fluctuation between independent cultures was incompatible with acquired immunity and matched the mutation hypothesis. From the relationship between the mutation rate and the distribution of survivors — in particular the fraction of cultures with no resistant cells at all — the authors could even estimate the rate at which the resistance mutation occurs per cell division. Resistance, they concluded, arises by spontaneous mutation before the virus is ever applied; the virus only selects the mutants already present.

[ … ]

The full paper develops the probability theory of the mutant distribution, tabulates the parallel-culture and single-culture data, and derives the mutation rate; it runs to about twenty pages and is available in full at the source below.

S. E. Luria & M. Delbrück · Genetics, vol. 28 · 1943