The effects of Bacillus subtilis on Caenorhabditis elegans fitness after heat stress

Microbes can provide their hosts with protection from biotic and abiotic factors. While many studies have examined how certain bacteria can increase host lifespan, fewer studies have examined how host reproduction can be altered. The nematode Caenorhabditis elegans has been a particularly useful model system to examine how bacteria affect the fitness of their hosts under different contexts. Here, we examine how the bacterium Bacillus subtilis , compared to the standard C. elegans lab diet, Escherichia coli , affects C. elegans survival and reproduction after experiencing a period of intense heat stress. We find that under standard conditions, nematodes reared on B .  subtilis produce fewer offspring than when reared on E .  coli .However, despite greater mortality rates on B .  subtilis after heat shock, young adult nematodes produced more offspring after heat shock when fed B .  subtilis compared to E. coli . Because offspring production is necessary for host population growth and evolution, the reproductive advantage conferred by B .  subtilis supersedes the survival advantage of E. coli . Furthermore, we found that nematodes must be reared on B .  subtilis (particularly at the early stages of development) and not merely be exposed to the bacterium during heat shock, to obtain the reproductive benefits provided by B .  subtilis . Taken together, our findings lend insight into the importance of environmental context and interaction timing in shaping the protective benefits conferred by a microbe toward its host.