Dr Susie Wood of the Cawthron Institute in Nelson describes research that aims to solve the mystery of the origin of tetrodotoxin. She presents three hypotheses: it comes from bacteria, it bioaccumulates in the food web or the slugs are producing it themselves. Susie gives an outline of some of the research being done and presents their conclusions so far.
Jargon alert: TTX – short form of the toxin tetrodotoxin.
DR SUSIE WOOD
We were really interested in finding out what was the origin, so where was the TTX coming from? There are three different hypotheses as to where this TTX might come from within the slugs. So the first one is produced by bacteria which lives within the slug. Then the second hypothesis is that it’s something that the slugs eat, so whether it’s some sort of algae that they’re eating or other organism and that they then accumulate the TTX, or the third option is that the slugs are producing it themselves.
Our current research suggests that the slugs are not accumulating the TTX from something they eat in the food chain. We’ve gone to sites where we know there is a really high abundance of slugs and we’ve had divers go down there and collected everything that the slugs might be eating and analysed all of them and found no high levels of TTX. So we know the levels in the slugs are really, really high, and so there’s got to be a reasonable level of that TTX in the organism that they’re eating, so we would be able to detect it if it was there.
The next thing we’ve been looking at is whether bacteria within the slugs produce TTX, so to do this, we’ve taken slugs with really high levels of TTX in them and we’ve tried to isolate and culture bacteria. We take different organs which we know have high levels, and we use a range of microbiology techniques and isolate out individual bacterial strains and then grow them up and test them for TTX. We’ve analysed about 200 different species now and found no indications of TTX.
Endogenous production is when an organism produces a toxin itself. Testing for endogenous production is really tricky to do. There are a range of different experiments that we can do, so we can do some experiments with the slugs in the lab and also some experiments with the non-toxic slugs, taking them to sites where there are toxic slugs, providing them with a non-toxic food source and just watching what happens to them.
There’s also now a range of really neat molecular techniques that are available. So one of the great things about this species is that we have both toxic and non-toxic populations. So potentially, we can use some of these populations to start looking for genes involved in toxin production. So when we know the levels are high, we can compare some of the genes that are present in the toxic slug versus those that are present in the non-toxic slugs, and we might, might be able to start to get some evidence that the slugs produce it themselves.
We feel fairly confident that it’s not something that the slugs are eating – we just haven’t seen any levels in any of the other organisms or food sources in the environment where the slugs live that would indicate that – so we’re still looking at whether it’s either bacterially produced or whether the slugs produce it themselves.