Sci. Aging Knowl. Environ., 25 August 2004
Longevity Is Infectious
Bacteria foster long life in young flies
R. John Davenporthttp://sageke.sciencemag.org/cgi/content/full/2004/34/nf78
Nasty germs in fast-food burgers can kill, but a more cordial relationship with bacteria fosters longevity, a new study reveals. Now, research suggests that microbes help shape life span: Young flies need bacteria in order to maximize their adult survival.
Bacteria deserve some of their bad reputation. Infections are a leading cause of mortality in elderly people, for instance. In addition, old nematodes apparently die when microbes overwhelm them, and long-lived mutant worms resist infection unusually well. But bacteria are good for animals too. They aid digestion and prime our immune systems to recognize more dastardly varieties. Furthermore, they are crucial for embryonic development, helping mammals establish healthy guts, for example. Brummel and colleagues wondered how bacteria influence the life span of the fruit fly Drosophila melanogaster.
Laboratory fly cultures typically contain bacteria. The researchers raised flies under axenic--microbe-free--conditions: They scrubbed fly eggs with alcohol, fed the animals irradiated food, and kept them in sterile containers. These insects died 30% earlier on average than did bacteria-laden flies. Next, they exposed axenic flies to bacteria at various stages of maturation. Insects inoculated with bacteria 2 days after hatching persevered. But nascent flies that remained in sterile conditions for 7 days died at the same rate as did flies that never encountered bacteria. To bolster the finding, the team bred flies under normal conditions and then switched them to antibiotic-laced chow at different times. When flies consumed antibiotics either 4 or 7 days after hatching, they lived long; any earlier and they perished young. A dietary change in the opposite direction--from fare that included antibiotics to "dirty" food--extended fly life on day 2 but not on day 4 or 7. Together, the results suggest that exposure to bacteria during the first several days of a fly's life establishes longevity.
Next, the team manipulated bacteria quantities late in fly life. They raised flies on normal food for 31 days and then served some of them antibiotic-rich broth. Insects on sterile food lived about 10% longer on average than did those that continued on unsanitary rations. The results suggest that toward the end of life, flies don't gain from exposure to bacteria but instead must resist infection.
Finally, the team tested the effect of diet on strains of flies with exceptional longevity. Lifelong antibiotic treatment did not shorten the lives of one strain, but it swatted another line down to normal survival times. The finding suggests that some mechanisms that stretch life span require the presence of bacteria, whereas others do not.
"It's truly groundbreaking," says symbiosis expert Margaret McFall-Ngai of the University of Wisconsin, Madison. Many biologists either ignore bacteria or view them as bad, she says, but this study "points out that there is a very important connection between environmental microbes and animals" that depends on host genetics. Future work should investigate whether immune molecules that recognize and dispose of harmful invaders also help foster positive relationships with bacteria, she adds. Such endeavors might illuminate how bacteria boost longevity and could help improve their image.
August 25, 2004
Suggested by Nick Bishop
Science of Aging Knowledge Environment. ISSN 1539-6150