| Method |
p.1099 left column bottom paragraph: "To follow a large number of cells inheriting the same pole and their progeny for many generations, [investigators] employed a high-throughput, continuous, microfluidic liquid-culture device that [they] built by using a standard soft-lithographic technique that others had developed for cell biology studies [refs 14, 15, 16 and 17]. [Their] device consists of a series of growth channels, oriented at right angles to a trench through which growth medium is passed at a constant rate (Figure 1A). This constant flow results in diffusion of fresh medium into the growth channels as well as removal of cells as they emerge from the channels into the main trench (Figure 1A)." |
| Comments |
P.1102 caption to fig.4: "Figure 4. Survival and Mortality-Rate Analysis
Showing Aging of E. coli (A) Survival curves of B/r, MG1655, and MG1655 lexA3 mutant (no SOS response). The pure exponential decay of the MG1655 lexA3 population with time allows us to directly extract 2.7% of the constant death rate per generation in the absence of the SOS response. The higher survival rate of MG1655 is due to the SOS response. The dotted horizontal line represents 50% decay of the initial populations. (B) Death rate computed by numerical differentiation of the survival curves in (A). Both B/r and MG1655 show increasing mortality rates, whereas MG1655 lexA3 shows a constant 2.7% rate of cell death." P.1100 right column bottom paragraph: "Because filamentation is a hallmark of the SOS response in bacteria, [investigators] asked how its suppression would affect [their] observation of filamentation. For this purpose [they] constructed an MG1655 derivative carrying a lexA allele, lexA3, whose protein product constitutively represses SOS gene expression even under conditions of DNA damage. Although the lexA3 mutant behaved virtually the same as MG1655 in terms of a constant growth rate, its filamentation rate, which was constant at approximately 1%, was significantly reduced, as expected. Note that B/r lacks sulA, a key SOS gene that inhibits cell division during the SOS response, and also shows a similar low filamentatin rate ( Figure S3). A more important difference between lexA3 and MG1655 is that, with a constant death rate of 2.7% per cell per generation, the population of the lexA3 mutant cells decayed exponentially ( Figure 4)." P.1101 left column: "These findings have important implications for the cause of cell death. That is, the death of the lexA3 mutant is random and requires the SOS response for survival. The much slower death rate of wild-type MG1655 cannot simply be due to a purely stochastic, age-independent fluctuation in DNA damage or metabolism otherwise, [investigators] would have observed an exponential decay like that of the lexA3 mutant ( Figure 4)." |