Evolutionary adaptation to temperature. VIII. Effects of temperature on growth rate in natural isolates of Escherichia coli and Salmonella enterica from different thermal environments

Abstract

Are enteric bacteria specifically adapted to the thermal environment of their hosts? In particular, do the optimal temperatures and thermal niches of the bacterial flora reflect seasonal, geographic, or phylogenetic differences in their hosts' temperatures? We examined these questions by measuring the relationship between the temperature-dependent growth rates of enteric bacteria in a free-living ectothermic host. We sampled two species of enteric bacteria (Escherichia coil and Salmonella enterica) from three natural populations of slider turtles (Trachemys scripta elegans) seasonally over two years. Despite pronounced differences in turtle body temperatures at different seasons and in different locations, we found no evidence that the thermal growth profiles of these bacteria mirrored this variation. Optimal temperatures and maximal growth rates in rich medium were nearly the same for both bacterial species (35-36°C, 2.5 doublings per hour). The thermal niche (defined as the range of temperatures over which 75% of maximal growth rate occurred) was slightly higher for E. coil (28.5-41.0°C) than for S. enterica (27.7-39.8°C), but the niche breadth was about the same for both. We also measured the thermal dependence of growth rate in these same bacterial species isolated from mammalian hosts. Both bacterial species had temperatures of maximal growth and thermal niches that were about 2°C higher than those of their respective conspecifics sampled from turtles; niche breadths were not different. These data suggest that these bacterial species are thermal generalists that do not track fine-scale changes in their thermal environments. Even major differences in body temperatures, as great as those between ectothermic and endothermic hosts, may result in the evolution of rather modest changes in thermal properties.