Published online 8 May 2008 | Nature | doi:10.1038/news.2007.360

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Bacteria 'can learn'

Colonies evolve to anticipate changes in their surroundings.

Ready to be eaten: bacteria can be taught to anticipate conditions in the stomach.Punchstock

The simple life of bacteria is a little less simple than you might have thought. New research shows that colonies of Escherichia coli can demonstrate a form of learning.

Such bacterial colonies can evolve the ability to anticipate changes in their immediate environment, say researchers led by Saeed Tavazoie at Princeton University in New Jersey. Other types of bacteria could too. This skill could give them the edge over other bacteria that merely adapt themselves to current conditions.

E. coli colonies, for example, can develop the ability to associate higher temperatures (as found in a human mouth, for example) with a lack of oxygen (as found inside the human gut). When exposed to higher temperatures, they alter their metabolism in anticipation of lowering oxygen levels. The results are published online in Science1.

The response is similar to Pavlovian conditioning, where after dogs learned to associate the sound of a bell with food, they would salivate on hearing the bell.

Learning by association

This result is unexpected, Tavazoie explains. "For as long as people have been studying the behaviour of bacteria, they have assumed that responses to environmental stimuli occur in an action–reaction fashion," he says. This concept, often known as homeostasis, has dominated the field for a century, he says.

"What we have found is that homeostasis is not the whole story," Tavazoie says. Bacteria can learn to respond in a way that is more complex than a simple reaction to current conditions; it anticipates future conditions.

It's the first evidence that bacteria have an ability for 'associative learning', Tavazoie adds. That's not to say, of course, that single-celled organisms learn in the same way as dogs, or people.

"Associative learning in dogs and humans happens over the course of the organism's lifetime, and involves modifications to the strength of connections between neurons in the brain," Tavazoie says. "The learning that we have discovered in bacteria occurs over a long evolutionary time-scale and involves changes in the connections between networks of genes."

Model behaviour

Tavazoie's team made the discovery by first using computer simulations to model the evolution of bacteria and see whether it might be possible for them to develop anticipatory behaviour.

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After discovering that this indeed seemed possible, they turned to live E. coli to see whether they could really show learning skills in the lab. They trained lab dishes of E. coli by shifting them from 25 °C to 37 °C and then from about 20% oxygen content to zero. They monitored responses over hundreds of generations. After a few weeks, the bacteria had 'learned' to anticipate the drop in oxygen by altering their metabolism just after the temperature change.

The new research could have implications for dealing with microbial infections and drug resistance, Tavazoie suggests. Anticipating the behaviour of microbes could also help to guide industrial processes that rely on them, such as brewing. 

  • References

    1. Tagkopoulos, I., Liu, Y.-C. & Tavazoie, S. Science advanced online publication, doi:10.1126/science.1154456 (2008).
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