The skin microbiome facilitates adaptive tetrodotoxin production in poisonous newts
Authors: Kevin R Theis
Patric M Vaelli
Heather L Eisthen
Janet E Williams
James A Foster
Lauren A O&#x27;Connell
Journal: eLife (2020)
Discipline: BIOLOGY & BIOCHEMISTRY
Keywords: TTX; amphibian; autoresistance; evolutionary biology; ion channel
Affiliations: Department of Integrative Biology, Michigan State University, East Lansing, United States., BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, United States., Department of Biochemistry, Microbiology, and Immunology, Wayne State University, Detroit, United States., Department of Animal and Veterinary Science, University of Idaho, Moscow, United States., Institute for Bioinformatics and Evolutionary Studies, University of Idaho, Moscow, United States., Department of Biology, Stanford University, Stanford, United States., Department of Biological Sciences, University of Idaho, Moscow, United States.

Abstract:  Rough-skinned newts (Taricha granulosa) use tetrodotoxin (TTX) to block voltage-gated sodium (Nav) channels as a chemical defense against predation. Interestingly, newts exhibit extreme population-level variation in toxicity attributed to a coevolutionary arms race with TTX-resistant predatory snakes, but the source of TTX in newts is unknown. Here, we investigated whether symbiotic bacteria isolated from toxic newts could produce TTX. We characterized the skin-associated microbiota from a toxic and non-toxic population of newts and established pure cultures of isolated bacterial symbionts from toxic newts. We then screened bacterial culture media for TTX using LC-MS/MS and identified TTX-producing bacterial strains from four genera, including Aeromonas, Pseudomonas, Shewanella, and Sphingopyxis. Additionally, we sequenced the Nav channel gene family in toxic newts and found that newts expressed Nav channels with modified TTX binding sites, conferring extreme physiological resistance to TTX. This study highlights the complex interactions among adaptive physiology, animal-bacterial symbiosis, and ecological context. 

Retracted: No

PubPeer comments:
 Thank you to the authors for this interesting paper. The authors state, in several places, that they isolated  TTX producing bacteria from newt skin. While I do not challenge the reliability of their LC-MS MS analysis detecting actual TTX from their cultures (that seems reliable), I do have to probe the authors about their conclusion of  production . Did the authors consider the possibility of sequestration, vs. production, of TTX? In other words, the bacteria that they isolated don t produce TTX, but rather have transporters to transport TTX into themselves (exposure to TTX via living on the newt skin), and were able to keep detectable TTX levels over the (non exhaustive) timeperiod used for establishment and growth of the pure cultures. Alternatively, did the authors attempt stable isotope feeding, to show labeling and therefore authentic production of TTX in their cultures? If not, I do not think the term  production , is warranted. Furthermore, there is indeed a long history of isolating TTX producing bacteria in marine systems, but that hasn t really seemed to  pan out . In other words, despite decades of reports there aren t reliable strains around that are producing TTX available in stock centers or across multiple laboratories today. Some suggestions for this have been, regulation of TTX production is quite specific hard to induce in the lab, and or plasmid encoding of TTX production that can be easily lost. Have the authors found that their TTX detection from these strains has been consistently measured at comparable levels? Or did it become lost lower over time? Lastly, are these 4 genera (7 strains) mentioned in the paper deposited in and or available from any stock centers? ,