Did you know even bacteria get old? Scientists traditionally assumed that bacteria were immortal, since these single-celled organisms split into two apparently identical daughter cells, which in turn divide, and so on. We now believe that this is not true. Eric Stewart of Northeastern University, and his colleagues took fluorescent images of individual E. coli cells over ten generations. Each generation the E. coli cells divide down the middle, giving each daughter cell one new tip and an old tip from its mother, or grandmother, or some older ancestor.
Photo From: Aging and Death in E. coli PLoS Biology Vol. 3, No. 2, e58 doi:10.1371/journal.pbio.0030058

Using computer software, Stewart et al. identified and tracked the tips of each bacterial cell. The results (open access: PLoS) indicated that the cell that inherits the old tip suffer a diminished growth rate, decreased offspring production, and an increased incidence of death.

More recently, Martin Ackermann and colleagues have published two papers on aging in bacteria in BMC Evolutionary Biology and Aging Cell (both open access). In the BMC Evolutionary Biology paper, Ackermann et al. evolved Caulobacter crescentus for 2000 generations under conditions where selection was strong early in life, but weak late in life. This selection had the effect of increasing the age of first reproduction and faster growth rates, but led to the unexpected evolution of slower aging. However, late acting deleterious mutations did invade and spread in populations.

In the Aging Cell paper, Ackermann et al. construct simple models to show why organisms might evolve aging, and test these models using age-specific performance data of C. crescentus to test the assumptions of the models. C. crescentus cell division is assymetric, and results in a sessile stalked cell and a motile swarmer cell. Ackermann et al.'s results showed that rate of cell division (hence fitness) declined with age for stalked cells, presumably because of the accumulation of damage in the stalked cell.

Naturally it would have been nice to see what happened to the motile cells, but unfortunately this data is quite difficult to obtain. Nonetheless, the implication is there that the assymetric cell division results in the partitioning of damage between 'parent" and "offspring" cells. At some point in the history of life, aging must have evolved (a wonderful resource is senescence.info).

Presumably the segregation of older material into one individual (termed 'parent') had to begin somewhere, and it would be nice to know the factors that led to this phenomenon. Bacteria may be the ideal model to explore this event. I'm looking forward to more studies in the near future.