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

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