Microbial custody: key microbiome inhabitant Sphingomonas alleviates silver nanoparticle toxicity in Daphnia magna.

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.