Evolution in the lab can tell us about stress resistance

What is it about?

The common bacterium Escherichia coli becomes very resistant to very low pH (high acid) if first exposed to a mild "acid shock" at a moderately low pH. For example, if you expose the bug to pH 5.5, it then becomes very resistant to pH 2.5, but if you put the bugs straight at pH 2.5, they die. We wanted to understand this in more detail so we evolved E coli in the lab. This was done by exposing it to very low pH, taking the few survivors, re-exposing them to a low pH, taking the survivors.....over and over again. Over the course of a few weeks, the cells became more resistant to low pH. We sequenced the complete genome of the evolved strains and found that all of them had a mutation in a particular protein that we showed is the switch that is turned on to cause acid resistance when cells are exposed to pH 5.5. Our mutant strains have this switch jammed in the "on" position - so cells think they are in mild acid even when they aren't.

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Why is it important?

Acid resistance is an important property of bacteria that helps them to survive passage through our gut, which is an important barrier to bugs that can cause disease. It can also help them to survive exposure to acid that may happen as part of food sterilisation. Finally, it could also be very useful to be able to engineer strains of bacteria that could grow at low pH for industrial processes that require a low pH. So the insights we gained from these experiments have potential applications for medicine, and for the food and chemical industries.

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