Adrien Sarlet

Biofilms are dynamic communities of microorganisms encased in a self-produced extracellular matrix. These resilient structures pose challenges across nearly all human activities, from healthcare to industry. The mechanical behaviour of a biofilm is shaped by the heterogenous composition of its matrix, both spatially and chemically. In turn, these mechanics influence the biofilm’s architecture at micro- and macroscopic scales, driving its complexity and adaptability. Morphologically, this is reflected in mechanical deformations of the biofilm known as wrinkles. In nature, biofilms often host different species of bacteria, allowing for a great diversity of matrix components. Here, we use a combination of two Escherichia coli strains as a model for a multispecies biofilm in which each bacterial strain produces one of two complementary matrix fibres: an amyloid protein (curli) or a polysaccharide (phosphoethanolamine-cellulose). Using fluorescence microscopy, we confirm that the two bacterial strains rapidly segregate into isogenic sectors, decreasing local heterogeneity. Furthermore, we show how wrinkles form, both in the homogenous central region of the biofilm, as well as at the boundary between sectors (i.e. where the two matrix producers co-localize). Finally, we show that increasing strain intermixing via the addition of bacteriophages results in thicker, taller wrinkles, irrespective of whether the two fibres are produced by two different strains or co-produced by the same bacteria.

Bacteria’s competition for nutrients and territory drives biofilm evolution (1 –4). The factors determining the outcome of competition among diverse bacterial species have a broad impact on a wide range of pathological (5), environmental (6), and microbiome interactions (7). While motility-related traits (8 – 11) and specific molecular mechanisms (12 , 13) have been identified as potential winning attributes in bacteria, a shared and universally conserved feature determining competition remains elusive. Here, we demonstrate that a simple morphological feature of individual bacteria, cell aspect ratio, provides a winning trait for the population. Using range expansion experiments (14), we show that relatively longer bacteria robustly conquer the expanding front, even when initially in minority. Using an agent based model of dividing bacteria, we reveal that the takeover mechanism is their emergent collective alignment: groups of locally aligned bacteria form "nematic arms" bridging the central region of the colony to the expanding front. Once at the front, bacteria align parallel to it and block the access of shorter bacteria to nutrients and space. We confirm this observation with single-cell experiments and further generalise our findings by introducing a generic continuum model of alignment-dominated competition, explaining both experimental and cell-based model observations. Moreover, we extend our predictions to spherical range expansions (15) and confirm the competitive advantage of being longer, even though the effect is less pronounced than in surface-attached colonies. Our results uncover a simple, yet hitherto overlooked, mechanical mechanism determining the outcome of bacterial competition, which is potentially ubiquitous among various bacteria. With the current advances in genetic engineering, varying aspect ratios can work as a simple tunable mechanism for the on-demand setting of the outcome of bacterial competitions with widespread implications for biofilm control.

Bacterial competition shapes community architecture, yet a universally conserved determinant remains elusive. We show that cell aspect ratio –a simple morphological feature– confers a competitive advantage. Using growth-based range expansion experiments, we show that longer bacteria conquer the expanding front, even when initially in minority. Using an agent-based model of dividing bacteria, to isolate the effect of aspect ratio, we reveal that the takeover mechanism is collective alignment: groups of locally aligned bacteria form “nematic arms” bridging the central region of the colony to the expanding front. Once at the front, bacteria align parallel to it and block shorter bacteria from access to nutrients and space. We confirm this observation with single-cell experiments and further generalise our findings by introducing a generic continuum model of alignment-dominated competition, explaining both experimental and cell-based model observations. Moreover, we extend our predictions to spherical range expansions and confirm the competitive advantage, even though the effect is less pronounced than in surface-attached colonies. Our results uncover a simple, yet hitherto overlooked, mechanical mechanism determining the outcome of bacterial competition, which is potentially ubiquitous among various bacteria. Current advances in genetic engineering enable aspect ratio tuning as a mechanism with broad implications for biofilm control.