Bacteria show their smarts?

Written byAlla Katsnelson

| 2 min read

Microbes may have the capacity for a type of learning generally attributed to higher organisms, suggests a linkurl:paper;http://www.sciencemag.org/cgi/content/abstract/1154456 published online in Science today (May 8). "We have to start to think about bacterial behavior in a more sophisticated way," said linkurl:Saeed Tavazoie;http://genomics.princeton.edu/tavazoie/web/homes.html of Princeton University, who led the study. Researchers have long assumed that microbes respond to changes in their environment in a simple, straightforward manner. If the osmolarity in your gut suddenly rises, for example, microbes populating it will tweak their own osmolarity intracellularly to match. But Tavazoie suspected that homeostasis wasn't the whole story, since microbial responses sometimes don't directly relate to a stimulus. For example, after experiencing a raise in temperature, bacteria often scale down linkurl:oxygen metabolism;http://www.the-scientist.com/news/display/23245/ -- a seemingly unrelated move. "We hypothesized that these non-homeostatic responses were adaptations not to the conditions themselves, but to what [the conditions] mean to the organism in its native habitat," Tavazoie said. If a certain stimulus, such as a temperature jump, is generally followed by a second stimulus, such as a drop in oxygen levels, he thought, perhaps microbes can "learn" to make an association between the two, and predict that the second stimulus will strike as soon as they're hit with the first. "If an organism is able to capture [that information], then it is able to respond in an anticipatory way," Tavazoie explained. "This is a sort of associative learning." To test the idea that bacteria respond to their environment through a form of learning, the group developed a computer program that simulated microbial evolution. "What we ended up seeing is that in this virtual ecology you have highly reproducible and rapid evolution of biochemical networks that can build internal models of their environment and predict what's going to happen," he said. The bacterium Escherichia coli travels from the outdoor environment to the mouth of a human host, where the temperature shoots up, and shortly thereafter down the gastrointestinal tract to the gut, where oxygen is harder to come by. In E coli undergoing such conditions, the researchers found that many of the genes down-regulated by temperature up-shift were the same as those down-regulated by oxygen down-shift, suggesting that the bacteria use the temperature spike as a signal predicting anaerobic conditions to come. When the researchers placed the microbes into a novel environment, where the relationship between temperature and oxygen was reversed, they saw strong selection pressure favoring organisms that managed to uncouple the link between the two responses. He stressed, though, that the "learning" exhibited by bacteria differs from the learning that occurs in higher animals in that "it takes place over evolution and involves changes in the linkurl:genetic network";http://www.the-scientist.com/2007/3/1/65/2/ rather than reflecting changes that occur over the course of an animal's life. Still, he said, "A better understanding of [bacteria's] behavioral outputs will help us to control their behaviors" -- with applications in, for instance, industry such as biofuel production, and manipulating disease resistance.

Bacteria show their smarts?

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