Spatial segregation and cooperation in radially expanding microbial colonies under antibiotic stress

Abstract

Antibiotic resistance in microbial communities reflects a combination of processes operating at different scales. The molecular mechanisms underlying antibiotic resistance are increasingly understood, but less is known about how these molecular events give rise to spatiotemporal behavior on longer length scales. In this work, we investigate the population dynamics of bacterial colonies comprised of drug-resistant and drug-sensitive cells undergoing range expansion under antibiotic stress. Using the opportunistic pathogen E. faecalis with plasmid-encoded ({beta}-lactamase) resistance as a model system, we track colony expansion dynamics and visualize spatial pattern formation in fluorescently labeled populations exposed to ampicillin, a commonly-used {beta}-lactam antibiotic. We find that the radial expansion rate of mixed communities is approximately constant over a wide range of drug concentrations and initial population compositions. Fluorescence imaging of the final populations shows that resistance to ampicillin is cooperative, with sensitive cells surviving in the presence of resistant cells even at drug concentrations lethal to sensitive-only communities. Furthermore, despite the relative invariance of expansion rate across conditions, the populations exhibit a diverse range of spatial segregation patterns, with both the spatial structure and the population composition depending on drug concentration, initial composition, and initial population size. Agent based models indicate that the observed dynamics are consistent with long-range cooperation, despite the fact that {beta}-lactamase remains cell-associated in E. faecalis, and experiments confirm that resistant colonies provide a protective effect to sensitive cells on length scales multiple times the size of a single colony. Furthermore, in the limit of small inoculum sizes, we experimentally show that populations seeded with (on average) no more than a single resistant cell can produce mixed communities in the presence of drug. While biophysical models of diffusion-limited drug degradation suggest that individual resistant cells offer only short-range protection to neighboring sensitive cells, we show that long-range protection may arise from synergistic effects of multiple resistant cells, even when they represent only a small fraction of a colonys surface area. Our results suggest that {beta}-lactam resistance can be cooperative even in spatially extended systems where genetic segregation typically disfavors exploitation of locally produced public goods.