Fig. 2
Competence is regulated by cell density. a, b Growth curves (OD595 nm) and competence expression measured as relative luminescence units (RLU) expressed from the promoter of the late competence gene ssbB for populations inoculated at a range of densities and grown in C+Y medium with initial pH 7.9. In a, cells were precultured in acid conditions (pH 6.8), while in b cells were precultured in non-acid conditions (pH 7.9). Four replicates are shown for each of seven inoculation densities (OD595 nm): 0.1 (green), 0.05 (red), 0.01 (blue), 10−3 (purple), 10−4 (light green), 10−5 (yellow), and 10−6 (brown). Competence does not develop in cells coming from acid preculture and inoculated at a density of 0.1. c Effect of inoculation density on the time until competence initiation (left panel) and the population density at which competence was initiated (right panel). Competence initiation was defined as the time where the RLU signal exceeded 200 units. Note that our luminometer has enough sensitivity to detect light from competent cells at a density of 1.56 ×10−3 or higher even if they correspond to a subpopulation (Supplementary Fig. 2). d Predictions of the mathematical model concerning the effect of inoculation density on the timing of competence initiation (left panel) and the density at which competence initiates (right panel). In the model, competence initiation was defined as the time where the total concentration of ComX times the population density exceeds 2000 units. Non-acid preculture is simulated in the model by setting the initial amount of all proteins in the competence regulatory network to the value they attain when cells are competent. e The model predicts that populations inoculated at lower densities will reach a threshold CSP concentration (dotted line) at a lower density than populations inoculated at higher densities
