Field of Science

Trojan horse predators

This post was chosen as an Editor's Selection for A while ago, Angry by Choice wrote a post about a fungi that kills its nematode prey by making little lasso ropes to catch the worm in. At the time, I thought there must be some exciting way that bacteria could cause wormy destruction, but it wasn't I read a paper from Lucas (reference below) that I actually found one.

It's not as visually exciting as the little fungi nooses, but it's just as chemically exciting. As bacteria are not capable of forming phyiscal structures to capture a worm, they make chemical ones, specifically volatile chemicals which diffuse easily and can be sensed by the worm.

In layman terms they smell good. They smell like food.

This is officially the best MS Paint picture I've ever done

Nematodes don't have a huge number of well programmed behaviours in their little nematode brains, but "move towards food" aka "positive olfactory chemotaxis" is one of the most robust and common behaviors. And the bacteria take full advantage of this. They secrete chemicals that are based on modified quorum sensing molecules (usually used for bacterial communication), which cause the worms to not only arrive where the bacteria are waiting, but also to happily gobble them up, after all they do smell like food.

Once eaten, the bacteria end up in the worms digestive tract, where they start to secrete enzymes - two digestive proteases called Bace16 and Bae16. Although previous work had assumed that these two proteases worked on the outside of the worms cuticle the paper used flourescent labeling studies to show that both Bace16 and Bae16 had their effects once inside the worm. Bace16 (labelled in red) and Bae16 (labelled in green) were both injected into an unsuspecting worm, which was then visualised every hour:

Images taken after 2hs, 5hrs, 8hrs and 24hrs - from the reference.

It's a little hard to see in the small picture above, but it is clear that the worm is getting ill, breaking apart, and finally just decomposing due to the action of the two proteases. The bacteria is not using the proteases to break into the worm, but to break out of it, digesting the worm in the process. This was further proved by making bacterial strains with the genes for Bace16 and Bae16 knocked out. Infection with the knockout strains led to far less virulent bacteria, and worms that survived for far longer.

It's an interesting new type of predation - a kind of Trojan Horse predator. Rather than chasing its prey, or directly infecting it, the bacteria gets itself eaten and then destroys the worm from the inside out. The paper suggests that as well as being interesting, this knowledge could help lead to more efficient biocontrol strategies for the elimination of nematode worms, now we know the active series of events involved in nematode predation by bacteria.

Niu Q, Huang X, Zhang L, Xu J, Yang D, Wei K, Niu X, An Z, Bennett JW, Zou C, Yang J, & Zhang KQ (2010). A Trojan horse mechanism of bacterial pathogenesis against nematodes. Proceedings of the National Academy of Sciences of the United States of America PMID: 20733068
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HHM said...

How gruesome, yet fascinating. What are the advantages for the bacteria in this relationship?

Lucas Brouwers said...

Another fine example of the numerous wicked ways of bacteria ;). And poor Odysseus - thinking he was being original!
I was wondering - are these guys actually dependent on the nematodes, or can they survive 'in the wild'?

Lab Rat said...

The bacteria in question is Bacillus nematocida (should probably have mentioned that in the post!) and it predates C. elegans. There doesn't seem to be any literature I can find on *why* it does this, possibly as a source of nutrients is my best guess.

It seems to have been isolated and found to kill worms in China in 2004 - so not a huge amount of work has gone into studying its motives - just on the important worm-killing ability.

Paul Orwin said...

This is a lovely story, thanks for pointing it out. I've been working on related stuff during my sabbatical this year. I think this area of symbiosis/pathogenesis is ready to take off. The most interesting things will be when people get away from c. elegans (the Balb/c mouse of nematodes) and start using wild species. As to the questions pointed out above, I still need to read the paper, but there are a couple of things to keep in mind.
1) C. elegans is not a soil nematode. Despite what people usually write in papers, C.elegans is not usually found in the soil (sometimes in compost heaps)
2) C. elegans is an indiscriminate bacteriovore. The way this interaction takes place in the soil, with more selective nematodes, may be quite different. Also, true soil nematodes may have defenses against this
3) Bacillus is a tough bug! they make spores, they have thick cell walls, they make lots of antibiotics and secondary metabolites, toxins, and biofilms. This activity may only be part of a larger life strategy.

Good stuff!

Lab Rat said...

@Paul: Thanks for the extra information! I do agree that work in this field needs to move away from the model organism of C. elegans, and look at more holistic whole biosphere interactions as well. Two-organism interactions might become more meaningful in the context of a whole system.