FIGURE 3. Cell-fate decision following infection.

(A) A schematic description of our cell-fate assay. Multiple fluorescently-labeled phages (green) simultaneously infect individual cells of E. coli. The post-infection fate can be detected in each infected cell. Choice of the lytic pathway is indicated by the intracellular production of new fluorescent phages (green), followed by cell lysis. Choice of the lysogenic pathway is indicated by the production of red fluorescence from the P_RE promoter, followed by resumed growth and cell division. The three stages of the process correspond to the three images seen in panel (B) below.

(B) Frames from a time-lapse movie depicting infection events. At time t = 0 (left), two cells are seen each infected by a single phage (green spots), and one cell is infected by three phages. At t = 80 min (middle), the two cells infected by single phages have each gone into the lytic pathway, as indicated by the intracellular production of new phages (green). The cell infected by three phages has gone into the lysogenic pathway, as indicated by the production of red fluorescence from P_RE (red). At t = 2 hr (right), the lytic pathway has resulted in cell lysis, whereas the lysogenic cell has divided.

(C) Scaled probability of lysogeny [f(m, l)]^1/m as a function of viral concentration (m/l). Data from different MOIs (filled squares, different colors) collapse into a single curve, representing the probability of lysogeny for each individual infecting phage (f₁), in a cell of length l infected by a total of m phages. f₁ can be fitted to a Hill function, f₁(m/l) = (m/l)^h/(K^h+(m/l)^h), with h ≈ 2.

(D) The probability of lysogeny as a function of the relevant input parameter, at the single-cell (top, blue; input is MOI of the individual cell) and population-average (bottom, green; input is the average MOI over all cells) levels. Circles: experimental data. Solid lines: theoretical prediction, fitted to a Hill function. The decision becomes more “noisy” (lower Hill coefficient, h ≈ 1) when moving from the single-phage (panel C) to the single cell level. Moving from the single cell to the population average does not decrease the Hill coefficient further.

Reprinted from (96), with permission from Elsevier.