Sci. Aging Knowl. Environ., 2 February 2005
Ageless No More
Once thought immortal, gut bacteria suffer aging's toll
They don't develop wrinkles in their cell walls or sprout gray flagella, but intestinal bacteria do grow old, new research suggests. The work shows that senior bacteria grow and reproduce more slowly than do their youthful counterparts, a result that defies long-held beliefs about which organisms decay over time.
Biologists have thought for decades that bacteria and other microorganisms that reproduce by splitting were immune to aging (see "Growing Old, Bacterial Style"). A bacterial cell parcels out its contents between its two offspring, and the bugs wouldn't last long if each cell inherited a load of worn-out molecules, researchers surmised. In the 1990s, scientists revisited this reasoning, noting that bacteria could age if bugs divide asymmetrically, allotting more newly made goodies to one offspring cell than to the other. For example, researchers measured declines over time in yeast "mother" cells that bud off smaller "daughters" and bequeath their progeny most of their undamaged proteins (see "Protective Parents"). But the intestinal bacterium Escherichia coli doesn't seem to meet the criteria for aging: Fresh and dated lipids, proteins, and DNA get allocated equally to each offspring cell. However, one 1963 experiment hinted that bacteria might deteriorate and that older bugs reproduce more slowly. Microbiologist Eric Stewart of INSERM in Paris and colleagues decided to follow up on this work to determine if they could detect signs of aging in the microbes.
As a bacterium splits, it makes two new poles, structures that cap each end of a cell. Each offspring receives one freshly minted pole and one old pole the parent cell inherited from the previous division. Like family heirlooms, existing poles pass from generation to generation, but the old poles in any two cells might have undergone a different number of divisions. The researchers followed a colony of E. coli and tracked the distribution of poles over successive generations. They contrasted each pair of cells produced in the eighth generation and found that the cell that inherited the older cap grew in size about 2% slower than its counterpart sporting the younger end did. Moreover, bacteria with fresher poles were also "born" larger and reproduced sooner. The results suggest that even E. coli cells get old. "Aging is more widespread than expected; it may be universal," says Stewart.
"It's the clearest demonstration of an age effect" in E. coli, says microbial geneticist Steven Finkel of the University of Southern California in Los Angeles. Why natural selection would allow the cells to deteriorate is a mystery, he says. Repairing every nick and scratch might be too costly, or the poles might serve as an aging clock that counts down a cell's life span, allowing younger, healthier bugs to replace older, dilapidated ones. The researchers hope to pin down what makes an older pole detrimental to cells, Stewart adds. It might be tainted with some harmful molecules, or it might lack beneficial ones. Finding the answer might clarify what makes bacteria act their age.
February 2, 2005
Science of Aging Knowledge Environment. ISSN 1539-6150