Thursday, November 3, 2011

Senescent bacteria and persisters

Persisters are small subpopulation of bacteria that are neither killed nor grown in the presence of antibiotics. Features of persister bacteria have already been discussed previously.

Integrating the features of persistence and bacterial senescence, Klapper et al. (2007) proposed that persisters are senescent bacteria. Their proposal is based on the model of bacterial senescence put forward by Stewart et al. (2005). As per the model, mother cells undergo gradual aging and have a reduced growth rate and finally stop dividing, whereas the daughter cell produced from a mother cell is a rejuvenated offspring capable of faster growth. Klapper et al. (2007) also made an assumption that the older cells are more tolerant to antibiotics than the younger cells due to their slow growth rate.
Thus, as per Klapper et al. (2007), when a bacterial culture is treated with bactericidal antibiotics, the younger cells are killed due to their fast growth rate whereas the older mother cells (persisters) survive. However, upon removal of antibiotics, the rejuvenated offspring produced from the mother cells quickly repopulate the culture. Since, during the exponential phase, the number of older cells is low, there may not be many survivors when antibiotics are used against exponential phase bacteria. Thus, Kappler et al. (2005) argued that senescence can explain all the features of persister cells. Their argument would have been correct if the current model of bacterial aging were true. However, the current model of bacterial aging may not be complete as I discussed previously.

One drawback with Stewart et al. (2005) model of aging is that it cannot explain WHY the presence of mother cells in a bacterial culture is advantageous to the whole population. As per this model, the older mother cells also give rise to rejuvenated offspring. If their model is correct, senescence is disadvantageous only to the individual mother cell but is advantageous to the population because old mother cells not only can give rise to rejuvenated offspring but also are resistant to antibiotics.

Stewart et al. (2005). Aging and death in an organism that reproduces by morphologically symmetric division. PLoS Biol 3(2), e45.
Klapper et al. (2007). Senescence can explain microbial persistence. Microbiology 153(Pt 11), 3623-30.

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