Microbial custody: key microbiome inhabitant <i>Sphingomonas</i> alleviates silver nanoparticle toxicity in <i>Daphnia magna</i>

Increased usage of nanotechnological applications inevitably leads to exposure of hosts and their associated microbiomes to metallic nanoparticles. Various bacteria within the microbiome harbour mechanisms to protect themselves against metal-related toxicity. These mechanisms have been broadly described in the absence of a host. Here, we studied how silver ion-resistant bacteria isolated from the Daphnia magna microbiome shape the host's exposure to silver nanoparticles. With germfree and mono-associated neonates, the effects of these microbes on the sensitivity of D. magna to silver nanoparticles were studied. By using this approach, a core member of the D. magna microbiome Sphingomonas yanoikuyae was identified to be silver-resistant. Neonates mono-associated with S. yanoikuyae were as sensitive to silver nanoparticles as naturally colonized neonates, whereas mono-association with Microbacterium and germfree neonates had increased sensitivity. Silver ions are the major attribution to toxicity in germfree and Microbacterium-associated neonates, whereas particles contribute more to the toxicity for the naturally- and Sphingomonas-colonized neonates. Sphingomonas accumulated in vivo more silver ions from its local environment than the other D. magna bacterial isolates. The current study shows that bacteria can play a vital role in shaping the speciation of nanomaterials and thereby modifying the toxicity to hosts.