Diffuse interactions between symbiotic bacteria and salamanders

A growing body of evidence indicates that all animals and plants have intimate associations (symbioses) with microbes. Scientific opinion has shifted from viewing microbes primarily as pathogens to the idea that healthy animals and plants carry specialized communities of coevolving microorganisms. However, the generality of this proposition is unknown because surveys rarely compare host-associated microbial assemblages with those of similar host species or free-living assemblages in relevant microhabitats. To evaluate the specificity and consistency of microbial associations in salamanders, I analyzed just over 3.6 million 16S ribosomal RNA gene sequences from paired samples from host-associated bacteria on the skin of 62 salamanders and freeliving bacteria from surfaces in their immediate home environments in the Great Smoky Mountains. I tested for community-level differences in relative abundance of bacterial taxa and population-level differences in haplotype frequencies within bacterial taxa. Comparing host-associated and free-living samples, I found evidence of bacterial specialization at both community and population levels, consistent with the idea that salamander skin is a selective medium supporting a distinctive microbial assemblage. However, I found no strong evidence of bacterial specialization among salamander species. Community differences among hosts were associated with spatial distance rather than host species or environmental similarity. Population-level differentiation among hosts was not apparent, consistent with frequent horizontal transmission. Salamander-bacterial associations are generally diffuse rather than specialized, with many host and symbiont taxa interacting in the same region.