Monday, August 6, 2007

Even bacteria get old....

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 INSERM, the French institute for health and medical research in Paris, 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. Using computer software, Stewart et al. identified and tracked the tips of each bacterial cell. The results 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, whom I met at the recent GRC Microbial Population Biology conference, 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 (I've covered some of this theory in an earlier post, but a wonderful resource is 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.

Photo From: Aging and Death in E. coli PLoS Biology Vol. 3, No. 2, e58 doi:10.1371/journal.pbio.0030058


  1. Really interesting. I, like most, had been taught that bacteria were immortal. This kind of aging makes sense, though. Thanks for this.

  2. Because of my background working with actinomycetes (Streptomyces and related bacteria), I was soon familiar with the idea that some bacteria have complex life cycles. Differentiation processes, resulting in distinct cell types (almost "tissues"!), are recognized in actinomycetes, myxobacteria and other groups. During the last years, programmed cell death has been described in bacteria (resembling eukaryotic apoptosis, but using different molecular mechanisms). The discovery of aging processes in E. coli (supposed to be a "simple" bacterium) goes one step beyond. Then, can we say that only virus are forever young???

  3. Good point. Viruses are produced fresh from cellular materials in each infection cycle. I guess that post-vacation numbness is wearing off:)

  4. Good work . Aging Caulaobacter are known to have longer stalks, thus the size of the vegetative cell naturally reduces. It would be interesting to study the aging process in these bacteria . What about dormant cells?? Or spores for that matter??