Two species of bacteria share a single Petri dish. Both eat the same food, and by all rules, one should outcompete the other. But in reality, they often live together for years. The secret lies in knowing when to leave.
Bacteria don't stay put. When a stranger appears nearby, they start moving faster. Not immediately, though: first slowly, then, when the strangers become numerous, abruptly, as if an alarm goes off. Scientists call this 'fleeing from a neighbor' and tested it in a computer model.
It turns out that this behavior spontaneously creates order. Bacteria of one species gather in patches, and those of the other in different patches. The patches don't mix, and each species gets its own share of food. Even the weaker species, which would lose in a well-mixed environment, now survives. Sometimes it even becomes more abundant than the strong one.
So a simple rule—'speed up when a stranger is near'—explains why so many species live side by side in nature without destroying each other.
as written by the authors arXiv:2601.13947
Bacteria regulate their motility through a variety of mechanisms, including quorum sensing (QS) and other density-dependent responses mediated by diffusible signals. While nonlinear density-dependent motility is well known in active-matter theory to generate nonequilibrium spatial patterns, its consequences for the coexistence of growing, interacting species remain less explored. Here we develop a minimal spatially structured model for two strongly competing species in which local demographic interactions are coupled to an escape response: each species increases its motility nonlinearly (sigmoidal) with the local abundance of its competitor. We show that this sigmoidal motility regulation promotes optimal spatial self-organization and can sustain long term coexistence via segregation, even in parameter regimes that yield competitive exclusion in well-mixed Lotka-Volterra dynamics. On two-dimensional lattices, the interplay between demographic competition and density-dependent motility generates a range of emergent patterns, including regimes in which the weaker competitor counterintuitively has higher total abundance. Overall, our results identify nonlinear, competitor-induced motility as a fundamental mechanism capable of sustaining coexistence in competing microbial populations.
In the model, the weak species sometimes becomes more numerous than the strong one — simply because it flees faster and grabs empty spots.