In studying antibiotic-resistant strains of bacteria, researchers from Boston University and the Wyss Institute for Biologically Inspired Engineering at Harvard have discovered that charitable behavior exists in one of the most microscopic forms of life.

The researchers found that the bacterial populations that were successful in withstanding doses of antibiotics were those in which a few highly resistant isolates sacrificed their own well being to improve the group's overall chance of survival. In the successful populations, the most antibiotic resistant isolates produce a small molecule called indole.

Indole seems to act as something of a steroid - allowing the strain's weaker members to bulk up enough to fight off the antibiotic attack. While indole may save the group as a whole, the production takes much out of the previously strong antibiotic resistant isolates that donate it.

"We weren't expecting to find this," said lead investigator James J. Collins, Ph.D., professor of Biomedical Engineering at Boston University and a core faculty member of the Wyss Institute. "Typically, you would expect only the resistant strains to survive, with the susceptible ones dying off in the face of antibiotic stress. We were quite surprised to find the weak strains not only surviving, but thriving."

The findings also demonstrate the high level of complexity within bacterial strains - which can have impact on the medical community. Until now, it was assumed that the overall resistance level of any given population was reflected in each of its isolates. Instead, Collins and his team found that dramatic differences can exist within a single population with some bacteria showing exceptional resistance and some almost none, not unlike cancer cells in humans.

"Now, when we measure the resistance in a population, we'll know that it may be tricking us," said Collins. "We'll know that even an isolate that shows no resistance can put up a stronger battle against antibiotics thanks to its buddies."

The findings are published in today's edition of Nature

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