by However, if the test tube is kept under conditions favorable to bacterial growth, the cloudy coating will reappear over time. This shows that the bacteria developed resistance to the phages and were able to multiply.
What role did the phages play in this change?
Some scientists thought that the phages encouraged the bacteria to mutate in order to survive. Others have suggested that bacteria mutate randomly, and that the development of phage-resistant variants was simply a matter of luck. Luria and Delbrück worked together for months to solve this problem, but none of their experiments were successful.
On the night of January 16, 1943, Luria received a clue about how to solve the mystery while watching a colleague hit the jackpot at a slot machine. The next morning, he hurried to his laboratory.
Luria’s experiment consisted of a few tubes and dishes. Each tube contained a nutrient broth that would help the bacteria E. coli multiply, and each dish contained material coated with phages. A few bacteria were placed into each tube and given two opportunities to generate phage resistant variants. They could mutate either in the tubes in the absence of phages, or they could mutate in the dishes in the presence of phages.
The next day, Luria transferred the bacteria in each tube into a dish filled with phages. The next day, he counted the number of resistant bacterial colonies in each dish.
If bacteria develop resistance to phages by interacting with them, none of the bacteria in the tubes should have mutations. On the other hand, only a small number of bacteria – say, 1 in 10 million bacteria – should spawn resistant variants when transferred into a dish containing phages. Each resistant variant in bpage would grow into a colony, but the other bacteria would die from infection.
If bacteria develop resistance independent of interaction with phages, some of the bacteria will have mutations in the tubes. This is because every time a bacterium divides in a tube, there is a small probability that it will spawn a resistant variant. If the initial generation of bacteria is the first to mutate, at least half of the bacteria will be resistant in future generations. If a bacterium in the second generation is the first to help, at least an eighth of the bacteria will be resistant in future generations.
Like cash loans with small prizes in slot machines, late-generation mutations occur more frequently but produce fewer resistant variants. Like pots of gold, early generation mutations are rare but give a large number of variants. Early generation mutations are rare because only a small number of bacteria are available early to mutate.
For example, in a 20-generation experiment, a mutation occurring at the 10th generation of bacteria would produce 1,024 phage-resistant variants. A mutation occurring at the 17th generation would produce only four phage-resistant variants.
The number of resistant colonies in Luria’s experiments showed a pattern similar to that of slot machine payouts. Most of the dishes contained no or small numbers of mutant colonies, but a few contained large numbers of mutant colonies that Luria considered grand prizes. This meant that the bacteria developed resistant variants before they interacted with the phages in the dishes.
Inheritance of experiments
Luria sent a note to Delbrück after completing his experiment, asking him to check his work. The two scientists then worked together to write a classic paper describing the experimental protocol and theoretical framework for measuring bacterial mutation rates.
Other scientists performed similar experiments by substituting penicillin and tuberculosis drugs for phages. Likewise, they discovered that bacteria did not need to encounter an antibiotic to become resistant to it.
Bacteria have relied on random mutations to cope with harsh, ever-changing environments for millions of years. As a result of their relentless random mutations, they will inevitably develop versions resistant to future antibiotics.
Drug resistance is a reality of life that we will have to accept and continue to fight against.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
It was written by: Qi Zheng, Texas A&M University.
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Qi Zheng does not work for any company or organization that would benefit from this article, does not consult with, own shares or be funded by any company or organization that would benefit from this article, and has not disclosed any affiliation relevant beyond their academic appointment.
