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

The microbiome buffers tadpole hosts from heat stress: a hologenomic approach to understand host-microbe interactions under warming

Phenotypic plasticity is an important strategy that animals employ to respond and adjust to changes in their environment. Plasticity may occur via changes in host gene expression or through functional changes in their microbiomes, which contribute substantially to host physiology. Specifically, the presence and function of host-associated microbes can impact how animals respond to heat stress. We previously demonstrated that 'depleted' tadpoles, with artificially disrupted microbiomes, are less tolerant to heat than 'colonized' tadpoles, with more natural microbiomes. However, the mechanisms behind these effects are unclear. Here, we compared gene expression profiles of the tadpole gut transcriptome, and tadpole gut microbial metagenome, between colonized and depleted tadpoles under cool or warm conditions. Our goal was to identify differences in host and microbial responses to heat between colonized and depleted tadpoles that might explain their observed differences in heat tolerance. We found that depleted tadpoles exhibited a much stronger degree of host gene expression plasticity in response to heat, while the microbiome of colonized tadpoles was significantly more heat sensitive. These patterns indicate that functional changes in the microbiome in response to heat may allow for a dampened host response, ultimately buffering hosts from the deleterious effects of heat stress. We also identified several specific host and microbial pathways that could be contributing to increased thermal tolerance in colonized tadpoles including amino acid metabolism, vitamin biosynthesis and ROS scavenging pathways. Our results demonstrate that the microbiome influences host plasticity and the response of hosts to environmental stressors.

Coevolution's conflicting role in the establishment of beneficial associations

Reciprocal adaptation between hosts and symbionts can drive the maintenance of symbioses, resulting in coevolution and beneficial genotypic interactions. Consequently, hosts may experience decreased fitness when paired with nonsympatric partners compared to sympatric symbionts. However, coevolution does not preclude conflict-host and symbiont can act to advance their own fitness interests, which do not necessarily align with those of their partner. Despite coevolution's importance in extant symbioses, we know little about its role in shaping the origin of symbioses. Here, we tested the role of coevolution in establishing a novel association by experimentally (co)evolving a host with a protective bacterium under environmental stress. Although evolution in the presence of nonevolving bacteria facilitated host adaptation, co-passaged hosts did not exhibit greater adaptation rates than hosts paired with nonevolving bacteria. Furthermore, co-passaged hosts exhibited greater fecundity when paired with sympatric, co-passaged bacteria compared to co-passaged bacteria with which they did not share an evolutionary history. Thus, shared evolutionary history between the hosts and microbes actually reduced host fitness and has the potential to impede evolution of new beneficial associations.

Probiotic microbes: Are their anti-melanogenicity and longevity promoting activities closely linked through the major "pathogenic" kinase PAK1?

PAK1-deficient mutant of C. elegans lives 60% longer than the wild-type. Interestingly, PAK1-deficient mutant of melanocytes produces less melanin (only a half compared with the wild-type) in the presence of either serum (PDGF) or α-MSH (alpha-melanocyte stimulating hormone). These observations indicate that the major "pathogenic" kinase PAK1 is responsible for both shortening the healthy lifespan, and PDGF/α-MSH-dependent melanogenesis. For screening of PAK1-blocking probiotic bacteria or their products, their anti-melanogenic as well as longevity promoting properties were examined. Recently it was found that C. elegans fed with Lactobacillus rhamnosus in Xinjiang cheese lives 40% longer than the worm fed with the standard E. coli. Interestingly, a Chinese traditional medicine called "ChiBai" fermented with the Lactobacillus rhamnosus also inhibited the α-MSH-induced melanogenesis, and this bacteria itself produces butyric acid that blocks the oncogenic HDAC (histone deacetylase)-PAK1 signaling pathway. These findings strongly suggest, if not proven, that anti-melanogenic activity of Lactobacillus and many other probiotic bacteria might serve as a reliable indicator for their longevity promoting activity. In this context, a popular Japanese Lactobacillus-fermented milk drink called "Calpis", developed a century ago, and recently proven to inhibit the melanogenesis by suppressing the PAK1-dependent tyrosinase gene expression, may potentially prolong our healthy lifespan.

Bacillus subtilis var. natto increases the resistance of Caenorhabditis elegans to gram-positive bacteria

AIMS

This study aimed to investigate the effect of Bacillus subtilis var. natto on the susceptibility of the model host, Caenorhabditis elegans, to bacterial infection.

METHODS AND RESULTS

Caenorhabditis elegans worms were fed with a standard food consisting of Escherichia coli OP50 strain (control) or B. subtilis (natto) during their larval stage. The worms were then infected with pathogenic bacteria. We analyzed their survival time and RNA sequencing-based transcriptome. Upon infection with Staphylococcus aureus and Enterococcus faecalis, the survival time of B. subtilis (natto)-fed worms was longer than that of the control. Transcriptome analyses showed upregulation of genes associated with innate immunity and defense response to gram-positive bacteria in B. subtilis (natto)-fed worms.

CONCLUSIONS

Bacillus subtilis (natto) conferred an increased resistance of C. elegans to gram-positive bacteria. Our findings provided insights into the molecular mechanisms underlying B. subtilis (natto)-regulated host immunity and emphasized its probiotic properties for preventing and alleviating infections caused by gram-positive bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

To the best of our knowledge, this is the first study to show that B. subtilis (natto) confers specific resistance against gram-positive bacteria.

Association with a novel protective microbe facilitates host adaptation to a stressful environment

Protective symbionts can allow hosts to occupy otherwise uninhabitable niches. Despite the importance of symbionts in host evolution, we know little about how these associations arise. Encountering a microbe that can improve host fitness in a stressful environment may favor persistent interactions with that microbe, potentially facilitating a long-term association. The bacterium Bacillus subtilis protects Caenorhabditis elegans nematodes from heat shock by increasing host fecundity compared to the nonprotective Escherichia coli. In this study, we ask how the protection provided by the bacterium affects the host's evolutionary trajectory. Because of the stark fitness contrast between hosts heat shocked on B. subtilis versus E. coli, we tested whether the protection conferred by the bacteria could increase the rate of host adaptation to a stressful environment. We passaged nematodes on B. subtilis or E. coli, under heat stress or standard conditions for 20 host generations of selection. When assayed under heat stress, we found that hosts exhibited the greatest fitness increase when evolved with B. subtilis under stress compared to when evolved with E. coli or under standard (nonstressful) conditions. Furthermore, despite not directly selecting for increased B. subtilis fitness, we found that hosts evolved to harbor more B. subtilis as they adapted to heat stress. Our findings demonstrate that the context under which hosts evolve is important for the evolution of beneficial associations and that protective microbes can facilitate host adaptation to stress. In turn, such host adaptation can benefit the microbe.

Temperature stress induces mites to help their carrion beetle hosts by eliminating rival blowflies

Ecological conditions are known to change the expression of mutualisms though the causal agents driving such changes remain poorly understood. Here we show that temperature stress modulates the harm threatened by a common enemy, and thereby induces a phoretic mite to become a protective mutualist. Our experiments focus on the interactions between the burying beetle Nicrophorus vespilloides, an associated mite species Poecilochirus carabi and their common enemy, blowflies, when all three species reproduce on the same small vertebrate carrion. We show that mites compete with beetle larvae for food in the absence of blowflies, and reduce beetle reproductive success. However, when blowflies breed on the carrion too, mites enhance beetle reproductive success by eating blowfly eggs. High densities of mites are especially effective at promoting beetle reproductive success at higher and lower natural ranges in temperature, when blowfly larvae are more potent rivals for the limited resources on the carcass.