Lecture 46: Bacterial Evolution Flashcards
What did Lamarck say about evolution?
- That life is not fixed, and that if an organism uses something more, its population size will decrease (giraffes) and vice versa.
- Change is directed by the environment
- For example, he believed that giraffes stretched their necks to reach higher branches, which eventually became a permanent feature of the species
What did Darwin say about evolution?
- That change is spontaneous (happens randomly) and is driven by natural selection.
- Basically, change is random and then selected for if it gives a survival advantage.
When did Darwin publish ‘origin of species’?
In 1859
At the time was Darwinian evolution accepted by the scientific community?
- It was accepted by the majority of scientists by mid twentieth century
- Except for those who beleived in Lysenkoism.
- Lysenkoism was a political campaign and pseudo-scientific theory that opposed genetics and science-based agriculture in the Soviet Union in the mid-20th century.
- Also, certain people thought that prokaryotes were an exemption to evolution
What was the ‘evidence’ for lamarckian evolution?
Observation: Add a toxic agent to bacterial culture and the entire culture becomes resistant
Interpretation: the agent makes the cells resistant (environment)
Conclusion: Bacteria unlike higher organisms follow Lamarckian evolution
What experiment was used to test the idea of Lamarckian evolution?
The Luria-Delbrück experiment
What hypotheses were tested by the Luria-Delbrück experiment?
Darwinian: Random mutations predicts that mutants appear in culture prior to adding selective agent
Lamarckian: Directed change predicts that mutants appear in the culture only after adding the selective agent
Describe the Luria-Delbrück experiment
- Different E.coli cultures were grown
- Aliquots from each tube were plated onto agar plates containing T1 phage, which normally kills E. coli.
- Any colonies that grew on these plates were resistant to the phage (Tonʳ phenotype).
- Some plates had many resistant colonies, while others had very few.
- This indicated big variation in the number of resistant colonies between plates.
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What conclusions can be made from the Luria-Delbruck experiment?
- If resistance occurred due to exposure to T1 phage, we would expect a consistent number of resistant colonies across all plates.
- Instead, the large variation in resistant colonies supports the idea that mutations happened randomly before exposure to the phage, not because of it.
- This supports Darwinian evolution, showing that mutations arise spontaneously, and natural selection allows resistant bacteria to survive when exposed to the virus.
Whilst the results of the ‘old data’ did appear to show that adding a toxic agent to a bacterial culture makes the entire culture become resistant, the discussion is what’s important for understanding the reasoning behind this.
Whilst the results of the ‘old data’ did appear to show that adding a toxic agent to a bacterial culture makes the entire culture become resistant, the discussion is what’s important for understanding the reasoning behind this.
what was the point of the newcombe experiment?
The Newcombe experiment (1949) was a follow-up to the Luria-Delbrück experiment, designed to further test whether bacterial resistance mutations occur before or after exposure to a selective pressure (like a virus or antibiotic).
Describe the Newcombe experiment (1949)
- E. coli bacteria were spread onto agar plates before exposure to a T1 phage.
- This was done under 2 conditions. In one condition, the plates were left untouched (control).
- In the other, the plates were gently spread again after some time, redistributing the bacterial colonies.
- T1 phage was then added to all plates.
What were the results of the the Newcombe experiment (1949)?
The plates that were left untouched had fewer resistant colonies.
The plates that were spread again had more resistant colonies, even though both were exposed to the phage at the same time.
This suggested that resistant mutants were already present before phage exposure, but in the untouched plates, they were initially clustered together and didn’t spread across the plate.
This reinforced the Luria-Delbrück hypothesis that mutations arise spontaneously and randomly, not in response to selective pressure.
It showed that resistant mutants existed before the phage was added; they just needed the opportunity to spread.
what was the point of the Lederberg and Lederberg experiment - 1952
- The Lederberg and Lederberg experiment (1952) further confirmed that mutations occur randomly rather than as a direct response to environmental pressures. It built on the Luria-Delbrück and Newcombe experiments but introduced a new technique: replica plating.
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Is replica plating a common technique used in microbiology and bacteria genetics?
Yes
Describe the 1952 Lederberg and Lederberg experiment?
- E.coli was grown on a normal plate that didn’t have antibiotics. The bacteria grew into colonies, some of which already contained random mutations for antibiotic resistance.
- A piece of muslin cloth was gently pressed onto the original plate, picking up bacteria from each colony. This was then stamped on to several new plates, some of which contained antibiotics.
- Some colonies survived on the antibiotic plates, showing they were resistant. But crucially, the same colonies were resistant across all antibiotic plates, proving they had mutated before exposure.
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What conclusions could be made from the Lederberg and Lederberg experiment?
- Mutations happened randomly before exposure to antibiotics, not because of exposure.
- Antibiotics select for pre-existing resistant mutants rather than causing resistance to develop.
- This strongly supported Darwinian evolution (natural selection) over Lamarckian evolution (adaptation to the environment).
So why are the resistant colonies always found in the same place?
Because the resistance mutation happened before the replica plating
Key messages from lecture
- Bacteria evolve through mutation
- Evolution is caused by selection of the fittest
- Check yourself the next time you say “the xxx evolved this trait because….”
What you really mean is “this trait survived because……” - Note the scientific method of gathering evidence from multiple experiments
Change is random and you select for it in your environment
Change is random and you select for it in your environment
