Stewart et al. (2005) model of replicative senescence in E.coli

Perhaps, the earliest evidence of replicative senescence in bacteria was provided by Liu (1999). By tracking the bacterial growth in liquid media with high viscosity, Liu (1999) observed the unidirectional growth and reproduction of E. coli. He proposed that the bacterium has an intrinsic cell polarity with one end behaving as a mother compartment and the other end as the daughter compartment resulting in the formation of two bacteria of succeeding generations. His model defined bacterial age by its experienced chronological time. Based on this model, he predicted that, on bacterial division, the old strand of DNA remain with the mother bacterium whereas the new strand goes to the daughter bacterium and that this distribution of old and new strands of DNA between the mother and daughter cells is responsible for the intrinsic differences between the two.

Later, Stewart et al. (2005) studied the senescence in E. coli using automated time lapse microscopy by following repeated cycles of reproduction. They followed individual exponentially growing cells up to nine generations of growth and reproduction. Their findings were comparable to those reported previously by Liu (1999). The bacterium exhibits cell polarities which give rise to an old pole with a reduced growth rate and a new pole with higher growth rate. They found that the average growth rate of old pole cells was 2.2% slower than that of new pole cells and that the new pole cells were larger and divided sooner than the old pole cells. In addition, the old pole cells were also more likely to die than the new pole cells. They concluded that the two apparently identical cells are functionally asymmetrical, with the old pole cell behaving as the aging mother cell and the new pole cell as the rejuvenated offspring.

Thus, according to these models, bacteria undergo aging and that 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.

Next- Population distribution of an E. coli colony- a comparison with Stewart et al. (2005) model

Liu, S. V. (1999). Tracking bacterial growth in liquid media and a new bacterial life model. Science in China 42, 644-654.

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