Hints of Altruism Among Bacteria

Gwyneth Dickey, Science News, September 1, 2010

When it comes to fighting antibiotics, E. coli bacteria have each other’s backs.

Just a few drug-resistant bacteria can release a protective substance that makes a whole population resilient to drugs, a new study in the Sept. 2 Nature shows. The findings may lead to new ways of combating drug-resistant germs in humans.

“Antibiotic resistance is a global health issue and we want to understand how and why bacteria become resistant to a particular antibiotic,” says biophysicist Hyun Youk of MIT.

It turns out that when E. coli, a common gut microbe that occasionally causes illness, becomes resistant to an antibiotic, it releases a molecule called indole to share with its more vulnerable neighbors. Indole is a molecule known to help E. coli tolerate stress. By sharing the compound, just a few members of the population can make the whole group of bacteria tolerant of an antibiotic.

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It turns out that the more resistant bacteria were producing indole. After further analysis, the team found that indole was turning on cellular pumps that push drugs out and was protecting against damage produced by chemically reactive free radicals.

When the researchers looked for genetic changes that might explain the indole production, they found numerous mutations that protected the bacteria against the antibiotic. There weren’t any for increased indole production. It seemed that resistant bacteria, untroubled by the antibiotic, were able to keep right on producing their normal levels of indole, sharing it with neighbors. In contrast, bacteria that aren’t drug resistant stop making indole in response to antibiotics.

But the indole production came at a cost to the resistant bacteria. Since they spent energy making the indole, they had fewer resources to use for their own growth and reproduction and grew more slowly than mutants that didn’t produce indole. Somewhat similar to the evolutionary idea of kin selection, the process may ensure survival of the mutants’ relatives.

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