Figure 4. Self-sensing in also apparent in the Rap-Phr system.

(a) Co-culture response ratio of PhrP secreting (rapP⁺;phrP⁺) and non-secreting (rapP⁺) strains as a function of YFP response of the latter strain. Shown are results with (dark green) or without (light green) the addition of conditioned medium collected from a ComX-producing E. coli. Response ratio of a control co-culture of two phrP⁺ secreting strains is also shown (gray). Each data point represents a separate measurement. Series of experiments over ≥3 varying optical densities were repeated ≥3 times, on different days. Line at response ratio of 1 represents the null hypothesis of no cell-autonomous response. (b) Fluorescence levels (mean ± st. err.) of a P_srfA-YFP reporter integrated into PhrP secreting (red) and non-secreting (blue) strains. Response was measured at very low densities with ComX conditioned medium added to the culture (methods). The response is measured for pure cultures of the two strains and in co-culture. Also shown is the auto-flurescence of a similarly measured wild-type with no YFP reporter (gray). Note that auto-fluorescence here is larger than that measured in Supplementary Fig. 7, due to the addition of conditioned medium. Asterisks and N.S. mark statistically significant and non-significant differences accordingly. All data points are given in Supplementary File 1.