Temporal dynamics of Spo0A control the B. subtilis cell fate decision of entering starvation or biofilm formation

B. subtilis ensures survival by differentiating into two distinct cell types. When facing mild starvation, cells activate the biofilm network. Conversely, under severe starvation, sporulation is triggered. Studies have shown that these mechanisms are activated by the same molecule, Spo0A. We investigate how Spo0A triggers these two mutually exclusive cell-fates using optogenetics, microscopy, flow cytometry, and mathematical modeling. Namely, using in-house engineered B. subtilis strains with optogenetic control of Spo0A and network modeling, we study how the amplitude, frequency, lower- and upper-bound of Spo0A oscillatory signals affect the ON-OFF bistability threshold of the biofilm network. Our results show a non-linear relationship in which decreasing the frequency causes an increase in the amplitude of the Spo0A oscillatory signal required to maintain biofilm formation. Additionally, for the same lower-bound (i.e., basal level) of Spo0A, the threshold to deactivate biofilm production is lower for higher frequencies of oscillation. In summary, our results provide evidence for why biofilm is predominantly active during the early stages of starvation, thereby contributing to the knowledge on how bacterial networks decode dynamical signals to activate stress-response networks.