Mutation rate dynamics reflect ecological changes in an emerging zoonotic pathogen

While mutation is often deleterious, it can also be adaptive. Mutation rates are therefore subject to a trade-off, and this might vary with both ecology and genome size. As bacterial pathogens must survive in challenging environments and often undergo genome reduction, both factors might lead them to evolve higher mutation rates. To investigate these predictions, we conducted mutation accumulation experiments on eight strains of the emerging zoonotic pathogen Streptococcus suis. Natural variation within this species allows us to compare tonsil carriage and invasive disease isolates, from both more and less pathogenic populations, with a wide range of genome sizes. We find that invasive disease isolates have repeatedly evolved mutation rates that are higher than those of closely related carriage isolates, regardless of variation in genome size. Independent of this variation in overall rate, we also observe a stronger bias towards G/C to A/T mutations in isolates from more pathogenic populations, whose genomes tend to be smaller and more AT-rich. Our results suggest that ecology is a stronger correlate of mutation rate than genome size over these timescales, and that transitions to invasive disease are consistently accompanied by rapid increases in mutation rate. These results shed light on the impact of ecology on the adaptive potential of bacterial pathogens.