BioEd Online

Fruit flies scrubbed clean of bacteria do not outlive their grubby siblings, according to researchers from the University of Southern California (USC), Los Angeles. The finding, published in this month's edition of Cell Metabolism¹, challenges a long-standing view that even harmless 'commensal' bacteria force the host organism to expend energy on their management, an obligation organisms find it harder to meet as they grow older.

The team, led by John Tower, compared fruit flies grown on standard fly food in six groups of twenty-five. Half of the groups contained flies that had been cleansed of bacteria as embryos and kept in bacteria free vials from then on; the other half had had no special treatment.

Every week a single fly was removed from each group and analysed for bacteria. The number of bacteria living in and on the normal flies increased with age, as expected. Among sterilised flies some bacteria were found occasionally, but the overall load was negligible compared to that of the normal flies. However, contrary to the researchers' expectations, there was no difference in life span between the two groups.

"We knew that bacterial flora increased with age and assumed that this was imposing a stress on the animal that would reduce its lifespan. Alternatively, if the bacteria were beneficial in some way, we would have expected the bacteria-free flies to live for a shorter period of time. We were surprised to find that bacterial presence had no effect on lifespan whatsoever," comments co-author Steven Finkel.

Feeling their presence

The results from this research contrast with those of a previous study showing that life span was affected by bacterial presence². "The outcomes of these sorts of experiments can depend greatly upon the species involved. I think the differences in the microbes could be responsible for what we are seeing," comments microbiologist Heidi Goodrich-Blair at the University of Wisconsin, Madison.

"There is a consensus that there are beneficial effects related to commensal microbes and that alterations to their populations within our bodies would be harmful. I'm surprised that removing bacteria from these flies didn't make their life spans plummet," comments evolutionary microbiologist Jonathan Eisen, from the University of California, Davis.

Whether or not these results would hold true for humans is a question that cannot be easily answered. Populations of bacteria in the gut do not dramatically increase with age in humans, but species abundances can change and inflammatory bowel disease may become more common as time goes on. However since humans depend on some of their bacteria for proper digestion, growing a human in a bacteria free environment would be unethical, as well as impractical.

But the new result from flies may still be relevant to human-aging research. The research team argues that it now knows that the presence of bacteria does not necessarily play a role in limiting life span, at least amongst flies. "This study takes us one step closer towards understanding the fundamental factors behind aging," adds Finkel. But Eisen is unwilling to go so far. "The research points out environmental conditions where bacteria don't matter, but that doesn't mean that it is true for all conditions," he says. "We should be cautious in our application of the data."