Spatial constraints determine the spread of plasmid-encoded antibiotic resistance between bacterial colonies

Plasmid transfer among bacteria is an important driver of the spread of antibiotic resistance. Surface-associated bacterial biomass is a hotspot for plasmid transfer due to the dense spatial packing of cells, but this biomass is often sparse (composed of discrete bacterial colonies). Compared to plasmid dynamics within a single colony, the determinants of plasmid transfer between discrete colonies are less understood. Yet, colonies routinely physically collide with each other as they grow and expand across surfaces. Here, we experimentally demonstrate that collisions between colonies of Stutzerimonas stutzeri and Escherichia coli enable the spread of an antibiotic resistance-encoding plasmid, with the extent of transfer determined by the spatial distance between bacterial inocula. To better understand how spatial constraints influence the mechanisms underlying inter-colony plasmid spread, we applied an individual-based model simulating plasmid dynamics between colliding colonies. Our simulations quantitatively predict how the probabilities of plasmid transfer and loss affect plasmid spread as colonies grow and collide. These effects are modulated by the distances between colonies and the spatial positioning of plasmid-carrying cells along the collision boundary. Our study reveals that inter-colony plasmid transfer is determined by the interplay between plasmid transfer, plasmid loss, and spatial constraints, expanding our understanding of plasmid dynamics in the spread of antibiotic resistance genes.IMPORTANCEThe spread of antibiotic resistance between spatially discrete microbial colonies is poorly understood, despite its relevance to persistent colonization on a variety of surfaces (e.g., medical devices, dental plaque, wound infections, indoor plumbing, etc.). Here, we combined experiments and individual-based modeling to show that physical collisions between growing colonies enable the spread of plasmids carrying antibiotic resistance genes. The extent of transfer depends on the initial spatial distance between colonies, the probabilities of plasmid transfer and loss, and the local spatial intermixing of plasmid-carrying and -free cells along the collision boundary. These findings reveal how the interplay between plasmid biology and microbial spatial organization governs the spread of antibiotic resistance and provide a quantitative framework for predicting plasmid dynamics in spatially structured environments.