Habitat fragmentation enhances microbial collective defence

Microbes often inhabit complex, spatially partitioned geometries such as host tissue or soil, but the effects of habitat fragmentation on microbial infection dynamics and ecology are poorly understood. Here we investigate how habitat fragmentation impacts a prevalent microbial collective defence mechanism: enzymatic degradation of an environmental toxin. Using a theoretical model, we predict that habitat fragmentation can strongly enhance the collective benefits of enzymatic toxin degradation. For the clinically relevant case where {beta}-lactamase producing bacteria mount a collective defence by degrading a {beta}-lactam antibiotic, we find that realistic levels of habitat fragmentation can allow a population to survive antibiotic doses that would far exceed those required to kill a non-fragmented population. This "habitat-fragmentation rescue" is a stochastic effect that originates from variation in bacterial density among different subpopulations and demographic noise. In contrast, the stochastic effects of habitat fragmentation are weaker in a model of collective enzymatic nutrient foraging. Our model suggests that treatment of a spatially complex, fragmented infection showing collective resistance may be far less effective than expected based on bulk population assumptions. This may help to explain lack of correlation between lab-measured antibiotic susceptibility values and clinical treatment success.