Tuesday, October 18, 2011
Experiment that could indicate that SCVs are senescent bacteria
One of the markers of senescence is the accumulation of oxidative carbonylated proteins. Cells of an E. coli population show asymmetry not only with respect to growth rate, but also with respect to protein oxidation levels (Desnues et al. 2003; Aguilaniu et al. 2003). An E. coli population consists of relatively low damaged daughter cells (low protein oxidation) that are reproductively competent and damaged mother cells with reduced reproductive ability (Desnues et al. 2003). In exponentially growing E. coli, the amount of protein aggregates increases over time and were found to be more prevalent in dead cells than in culturable cells (Maisonneuve et al. 2008a). Similarly, aggregated proteins accumulate in cells with older poles, which are associated with a decrease in reproductive ability (Lindner et al. 2008).
In the earlier blog, I had hypothesized that small colony variants of E. coli isolated using subinhibitory concentrations of aminoglycosides are senescent bacteria which are hypovirulent, slow dividing and form small colonies on solid medium. Measuring the levels of protein carbonylation can give an indication whether they are senescent bacteria. If they are senescent bacteria, one can expect the protein carbonylation levels to be high. As far as I know, level of protein carbonylation in small colony variants has never been measured (a search of “small colony variants” and protein carbonylation returned only two results in Google scholar).
However, increased protein carbonylation may not be conclusive evidence that SCVs are replicative senescent bacteria. Increased carbonylation can be a feature of both conditional and replicative senescence. Hence further research may be required to differentiate between these two. In fact, there is possibility that mutants that form SCVs (like hemin, menadione or thiamine mutants) may also show increased carbonylation which may be due to conditional senescence and not replicative senescence. Hence, I assume that both mutant SCVs and non-mutant SCVs may show increased protein carbonylation, the former due to conditional senescence and the latter due to replicative senescence.
If the yeast cells isolated by repetitively growing in early exponential phase have low number of bud scars (described earlier) and SCVs isolated using aminoglycosides have increased protein carbonylation levels, I can say with increased confidence that my model of bacterial aging is different from that proposed by Stewart et al. (2005) and that SCVs are senescent bacteria
Next- Why both slow and fast dividing subpopulation of bacteria are missed during routine culture?
Stewart et al. (2005). Aging and death in an organism that reproduces by morphologically symmetric division. PLoS Biol 3(2), e45.
Aguilaniu et al. (2003). Asymmetric inheritance of oxidatively damaged proteins during cytokinesis. Science 299(5613), 1751-3.
Desnues et al. (2003). Differential oxidative damage and expression of stress defence regulons in culturable and non-culturable Escherichia coli cells. EMBO Rep 4(4), 400-4.
Maisonneuve et al. (2008). Protein aggregates: an aging factor involved in cell death. J Bacteriol 190(18), 6070-5.
Lindner et al. (2008). Asymmetric segregation of protein aggregates is associated with cellular aging and rejuvenation. Proc Natl Acad Sci U S A 105(8), 3076-81.