Bacteria don't just crawl across a surface: they collectively decide what shape their colony will take. Picture a crowd moving across a square: each person steps on their own, but together they form a flow. Bacteria are the same: each moves individually, but together they create patterns of branching trails.
Scientists took Enterobacter sp. SM3 bacteria, which live in the gut and help heal its lining. They watched how the colony spread across a Petri dish under different temperatures, humidity levels, and surface stiffnesses. It turned out that conditions barely affect the future shape. What matters far more is how the colony's edge looks right now: its curves and protrusions determine where the bacteria will crawl next.
The researchers created a program called Morpher that tracks the colony's edge and predicts how it will spread. Even a tiny error in edge detection, less than a percent, led to a noticeable error in the prediction. This means the colony's fate is encoded in its boundary, not in its surroundings.
This knowledge could be useful in medicine: if we learn to read the edge of a bacterial colony, we can predict how microbes will colonize a wound or the gut, and intervene in time.
Enterobacter sp. SM3 bacteria help heal the intestinal lining, but they can also form biofilms that are hard to destroy.
In H.G. Wells's novel The War of the Worlds, Martian red weeds spread across Earth following invisible rules, just as bacterial colonies follow the shape of their edge.