Thursday, October 27, 2011

My model of bacterial senescence

The current model of bacterial senescence proposed by Stewart et al. (2005) suggests that as bacteria undergo aging, the growth rate of mother cell decreases with age whereas the daughter cell produced from the mother cell is a rejuvenated offspring with high growth rate which helps to maintain the bacterial lineage. In this case, mother cell retains the damaged proteins to itself whereas daughter cells are spared from accumulating damaged proteins.

In my model of bacterial senescence, different subpopulations of bacteria are considered. A bacterial colony comprises many subpopulations which may exhibit different growth rates. These subpopulations can be broadly divided into three - a small subpopulation of fast dividing young bacteria, a major population with intermediate growth rate and a small subpopulation of slow dividing bacteria that are towards the terminal stages of senescence.

The young bacterial population divides fast and is hypervirulent and may not contain any damaged proteins. After many divisions, the mother cell may gradually accumulate some damaged proteins whereas the daughter cells are rejuvenated offsprings. Thus a small population of young virgin bacteria is always maintained.

However, the majority of the population in a colony exhibit intermediate growth rate. Since they comprise the major population, the growth rate of the colony will be similar to the growth rate exhibited by this population. They may carry some amount of damaged proteins which are not segregated to mother cell alone, but are also transferred to daughter cells. However, with increasing amount of damaged proteins in mother cells, the daughter cells will also accumulate more damaged proteins.

Towards the terminal stages of senescence, mother cells may lose their ability to retain the damaged proteins and hence may be shared almost equally with daughter cells. At this stage, mother cell will also give rise to senescent daughter cells. Since both mother and daughter cells are old cells and divide slowly, a single senescent cell gives rise to small colony.

In a population, majority of the cells have only limited carbonylated proteins. This, along with a subpopulation of virgin cells without any damaged proteins, help to maintain a pool of young cells that are large enough to prevent the extinction of the population.

Thus, my model of bacterial aging is close to that of Schizosaccharomyces pombe, the fission yeast (discussed in the blogpost on Sep.6). The similarity may not be surprising given that both bacteria and S. pombe divide by binary fission.

Next- Senescent bacteria and persisters

Stewart et al. (2005). Aging and death in an organism that reproduces by morphologically symmetric division. PLoS Biol 3(2), e45.

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