Toxin-producing bacteria can make this newt deadly

Some newts living in the western United States are poisonous, perhaps thanks to bacteria living on their skin.

Rough-skinned newts use tetrodotoxin — a paralytic neurotoxin also found in pufferfish and the blue-ringed octopus — as a defense against predators. But rather than making the toxin on their own, the amphibians (Taricha granulosa) may rely on microbes to produce it for them, researchers report April 7 in eLife. It is the first time that researchers have found tetrodotoxin-producing bacteria on a land animal.

Tetrodotoxin, or TTX, prevents nerve cells from sending signals that tell muscles to move (SN: 6/26/14). When ingested in low doses, the toxin can cause tingling or numbness. High amounts can trigger paralysis and death. Some newts harbor enough TTX to kill several people.

Marine animals including pufferfish get TTX from bacteria living in their tissues or by eating toxic prey. It was unclear how rough-skinned newts acquire the lethal chemical. Previous work in 2004 had hinted that the newts didn’t have the toxin-producing bacteria on their skin. Newts also didn’t appear to get TTX through their diet, which led scientists to think that the animals might make the toxin themselves.

But TTX is a complicated molecule to make, says Patric Vaelli, a molecular biologist at Harvard University. It seemed unlikely that newts would be able to do it when no other known animal can.  

Vaelli, who led the study while at Michigan State University in East Lansing, and his colleagues revisited the bacterial-origin hypothesis by growing bacteria found on the newts’ skin in the lab and screening for TTX. The team found toxin-producing microbes from four groups, including Pseudomonas, members of which also produce TTX in pufferfish, the blue-ringed octopus and sea snails. Poisonous newts had higher amounts of Pseudomonas on their skin compared with a population of nontoxic rough-skinned newts from Idaho.

Though valuable, the study doesn’t necessarily “shut the book” on the idea that newts can also produce TTX, says Charles Hanifin, a biologist at Utah State University in Logan. Newts harbor some variations of the toxin that haven’t yet been found in bacteria. Researchers also still don’t know how bacteria make TTX, which makes it harder to form concrete conclusions about where it’s coming from, he says.   

But the finding adds a microbial player to an evolutionary arms race that pits newts against garter snakes (Thamnophis sirtalis). Some snakes living in the same regions as toxic newts have developed resistance, allowing the predators to feast on TTX-laden prey. It’s possible that in response, Pseudomonas bacteria become more abundant on newts over time to make the animals more toxic, and put evolutionary pressure back on snakes to evolve higher levels of resistance, Vaelli says.