  Biofilm communities of Bacillus subtilis bacteria have recently been shown to
exhibit collective growth-rate oscillations mediated by electrochemical
signaling to cope with nutrient starvation. These oscillations emerge once the
colony reaches a large enough number of cells. However, it remains unclear
whether the amplitude of the oscillations, and thus their effectiveness, builds
up over time gradually, or if they can emerge instantly with a non-zero
amplitude. Here we address this question by combining microfluidics-based
time-lapse microscopy experiments with a minimal theoretical description of the
system in the form of a delay-differential equation model. Analytical and
numerical methods reveal that oscillations arise through a subcritical Hopf
bifurcation, which enables instant high amplitude oscillations. Consequently,
the model predicts a bistable regime where an oscillating and a non-oscillating
attractor coexist in phase space. We experimentally validate this prediction by
showing that oscillations can be triggered by perturbing the media conditions,
provided the biofilm size lies within an appropriate range. The model also
predicts that the minimum size at which oscillations start decreases with
stress, a fact that we also verify experimentally. Taken together, our results
show that collective oscillations in cell populations can emerge suddenly with
non-zero amplitude via a discontinuous transition.
