Bacteria Mutate Much More Than Previously Thought

In the case of Escherichia coli, the bacteria studied, this is as much as 1,000 times higher than previously believed. The study, just published in Science,  also suggests that many more genes mutate during bacteria adaptation to a new environment than previously thought. Both results - a much higher rate of advantageous mutations and a bigger number of genes mutating - have important implications for studies in antibiotic resistance and also how bacteria develop the capacity to attack their host.

Natural selection – the basis of evolution - is the process by which some organisms are more capable of life and self-reproduction because they are better fitted to a particular environment. And mutations are the raw material of evolution, in the sense that they are the source of new characteristics which equip the adapted organism with a bigger or smaller chance of survival.

But mutations can be either beneficial or disadvantageous, and although beneficial mutations are the crucial force behind adaptation and survival to a new environment, disadvantageous mutations are much better studied.

There are several reasons for this, the first relies on the fact that while “bad” mutations are very easily seen – the organism tends to die and disappear – “good” mutations tend to have a very small effect in the overall adaptation of individuals and so are easily missed. The fact that beneficial mutations with big effects are rare is because when an organism is already living in a particular environment means that is already adapted to it, and so does not need (or can even go through) radical changes.

The second reason is because in big populations where organism reproduce very quickly and many beneficial mutations occur at the same time – and these are the only populations where the studies can be done, since only in them evolution occurs on a visible timescale - there is competition between organisms with different mutations. This means that in the end, those with the most beneficial mutations will be reproducing in higher numbers masking the other mutations – this process is called “clonal interference” - and leads to an underestimation of the mutation rate.

The difference in the study of Lidia Perfeito, Isabel Gordo and colleagues from the Institute Gulbenkian of Science in Lisbon, Portugal, is that they measured the mutation rate of Escherichia coli in many different sized populations, including some small enough to avoid clonal interference although big enough to avoid disadvantageous mutations to spread too easily and kill the population.

Through the comparison of these different size populations, which ranged from to 20,000 cells to 10 million, Perfeito, Gordo and colleagues reached an amazing conclusion: that Escherichia coli mutation rate was a thousand times bigger than previously predicted and that thousand of mutations were going overseen because “better” ones overtook them in the population.

Although these studies were done in Escherichia coli - a very common bacterium found in the intestine of vertebrates, including man – the research by the group of Portuguese scientists potentially applies to any bacteria and will be especially important in the study of disease-inducing bacteria.

The importance of Perfeito, Gordo and colleagues’ results resides in two facts: first the fact that they show that beneficial mutations in bacteria are much more common than previously predicted suggesting that bacteria can adapt both to anti-bacterial medication, but also to their host, much quicker than previously thought and second the fact many more bacterial genes are mutating than those seen in the population what can have implications for the way evolution is understood.

As the team leader explained to the Portuguese Agency of News Lusa, "This study is a substantial contribution to the understanding of a central problem in the theory of evolution and has important implications for public health, more specifically for the understanding of antibiotic resistance and the development of new medicines against bacteria."

Reference: Science - 317, 813 (2007), “Effects adaptive mutations in bacteria: high rate and small”