Resolving a Function for Bacterial Cell Shape: Curvature Enhances Caulobacter Crescentus Surface Colonization

Abstract

Each bacterial species has evolved a characteristic shape that is stably maintained, indicating that specific shapes provide bacteria with selective advantages in nature. Much is known about the mechanisms by which bacteria acquire different shapes, but the benefits of specific morphologies are largely unknown. To understand the function of cell shape we focused on the curved bacterium Caulobacter crescentus. Paradoxically, C. crescentus curvature is robustly preserved in the wild but straight mutants have no known disadvantage in standard laboratory conditions. Here we demonstrate that cell curvature enhances C. crescentus surface colonization in flow, promoting the formation of larger and denser microcolonies. Leveraging microfluidics to mimic its natural environment and single-cell imaging, we also determined the mechanism by which curvature provides this benefit. Hydrodynamic forces cause curved cells to arc, optimally orienting polar pili, reducing their distance to the surface as the cell grows, and thereby enhancing surface attachment. C. crescentus thus repurposes pilus retraction, traditionally used for surface motility, for localized surface attachment. The benefit of curvature is modulated by flow intensity, potentially explaining why freshwater Caulobacter species that typically experience moderate flow are often curved while closely related marine species that experience stronger flows are often straight. Thus, our findings provide a mechanistic understanding of the potential benefit of bacterial curvature and highlight the importance of studying bacteria in conditions that reproduce their natural habitats.