Bacteria show signs of aging
Camera tracks life span of daughter organisms.
Bacteria may not have to deal with grey hair and wrinkles, but they do appear to grow old. By following microbes with a camera, researchers have revealed aspects of their life cycle. Their innovation could help people investigate the molecular mechanisms involved in ageing.
Researchers studying ageing in cells focus on two key characteristics: asymmetric cell division and the stage of the life cycle leading up to reproduction (childhood, if you like). "It's been proposed that these two features are an integral part of ageing," says microbiologist Eric Stewart of INSERM, the French institute for health and medical research in Paris.
The bacterium Escherichia coli, however, lacks both. The single-celled organism splits into two apparently identical daughter cells, which in turn divide, and so on. As a result, many scientists believed that it, and similar bacteria, were immortal.
The bacteria's apparently symmetrical division makes it difficult to track cells. Stewart and his teammates tackled the problem by taking pictures of a group of rectangular E. coli cells every two minutes. Their complete record included over 35,000 cells. In the lab, E. coli reproduces in half an hour.
A custom-made computer program analysed the snapshots. The computer identified and tracked the tips of each bacterial cell.
E. coli divides down the middle, giving each daughter cell one newly regenerated tip. But the cell's other tip is passed down from its mother, or grandmother, or some older ancestor.
The bacteria inheriting the older end reproduced 1% more slowly than their counterparts with each cell division, the team reports in PLoS Biology1. Stewart’s group was unable to follow bacteria until death, as after ten generations the dish became too crowded to spot individuals. The researchers now aim to explore the mechanism behind this reproductive slowdown.
Single-celled organisms that divide asymmetrically, such as yeast and the bacterium Caulobacter crescentus, were recently also revealed to reproduce more slowly as they grow old. Discovering how to monitor bacterial lifespan may help us understand the genes that control human ageing, which are implicated in everything from weakened immunity to sagging skin. This decline is called senescence.
"Bacteria and other single-celled organisms have traditionally been considered poor models for ageing because we have not been able to observe senescence for an individual cell," says biologist Jonathan Visick of North Central College in Naperville, Illinois.
"It's a way of doing the experiment that we've not previously been able to do," says Visick, who has studied ageing in bacteria. "That is the value of this finding."
- Stewart E. J., et al. PLoS Biol. 3, e45 doi:10.1371/journal.pbio.0030045 (2005).
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