  Bacterial biofilms collectively develop distinct and ordered structures,
including fibers, bundles, and branches. Often, it is unclear how these
structural motifs convey specific advantages to bacterial strains under
challenging conditions. In oxygen-limited environments, dense bacterial
aggregates generally deplete oxygen, which leads to arrest of bacterial growth.
However, we observed that biofilm-forming Bacillus subtilis could use branching
patterns to escape from these crowded regions by tracking the oxygen gradient.
The process depends on the chain-forming ability of the biofilm. As a result of
collective branching triggered by bending and mechanical buckling, bacteria can
extend from the oxygen-depleted biofilm core to the oxygen available periphery.
Remarkably, these bacterial branches are strongly chiral and curve in a
clockwise direction. Our analysis revealed that the surface friction and axial
rotation of the twisting cell wall break left-right symmetry on solid surfaces
and drive chiral bending of bacterial chains. We further observed that the
chirality of individual branches could propagate across a large scale and shape
the macroscopic morphology of the colony under a limited spatiotemporal growth
profile. Taken together, our results provide new insights into how simple
physical interactions lead to bacterial collaboration and promote the survival
of biofilm-forming strains in challenging environments.
