Abstract
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.
Highlights
Two studies found that the soil bacterium, Bacillus subtilis, was able to increase C. elegans survivorship after heat shock rela‐ tive to exposure to the standard lab diet, Escherichia coli (Donato et al, 2017; Gusarov et al, 2013)
We found that while C. elegans interactions with E. coli resulted in greater host fitness compared to B. subtilis under standard conditions (Figure 4b), interactions with B. subtilis conferred significantly greater host fitness, via increased fecundity, after heat shock (Figure 4d)
Our survival results for heat shocked older adult nematodes sup‐ port those of previous studies, in that B. subtilis led to increased host survivorship compared to E. coli (Gusarov et al, 2013; Donato et al, 2017)
Summary
Eukaryotic hosts generally obtain fitness benefits through associa‐ tion with specific microbes. Harboring certain microbes can increase host protection from biotic and abiotic stresses, such as enemies or environmental changes, and can provide hosts with nutrients that they cannot obtain from their diet alone (Douglas, 2009; Feldhaar, 2011; Oliver, Smith, & Russell, 2013). Two studies found that the soil bacterium, Bacillus subtilis, was able to increase C. elegans survivorship after heat shock rela‐ tive to exposure to the standard lab diet, Escherichia coli (Donato et al, 2017; Gusarov et al, 2013) These studies found that B. subtilis nitric oxide (NO) production and biofilm formation in the host's gut resulted in elevated host lifespan postheat stress. Caenorhabditis elegans generally exhibit little to no fecundity after exposure to intense heat stress
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