Fig 2. A role of extracellular signaling and rpoS in the density-dependent survival kinetics.

(A) A role of extracellular signaling: a high density of exponentially-growing cells (N _CFU ≈ 7·10⁸/ml) was transferred to a fresh medium without glycerol. N _CFU of cells in the fresh medium (green triangles) decreases similarly to that from the previous experiment (solid blue squares, re-plotted from Fig. 1). Next, a spent medium was prepared from a culture of a high density of cells. N _CFU of cells at low density in the spent medium (green inverse triangles) decreases similarly to that from the previous experiments (solid red circles, re-plotted from Fig. 1). The results indicate that extracellular signaling does not play a role for the density-dependent kinetics. See the text for details. (B) A role of rpoS: Under starvation, N _CFU of the ΔrpoS strain (open symbols) decreases faster than that of the wild type strain (solid symbols, re-plotted from Fig. 1); compare the slope of the dotted line −μ₀ ^ΔrpoS (= −0.035 hr ^-1) and the slope of the dashed line −μ ₀ (= −0.018 hr ^-1). See also S6 Fig for N _CFU of other densities of the ΔrpoS strain. Importantly, in the low cell-density cultures (e.g., open red circles), the periods during which N _CFU is maintained are much shorter for the ΔrpoS strain (brown region) than for the wild type strain (green region); note that here the brown and green regions are approximately determined as regions where the survival kinetics does not follow exponentially decay. This indicates that rpoS plays an important role for the wild type strain to maintain N _CFU for extended periods of time in low density under starvation.