If you happen to be a nematode, worm-stars are probably your worst nightmare. One minute, you’re swimming around minding your own business. The next, you’ve been sucked into a wildly thrashing mass of your peers, all stuck to each other by their tails. There is almost no escape from this desperately flailing entity. Eventually, you will die here.
(Video courtesy of University of Oxford)
Worm-stars are caused by a strain of bacteria known as Verde1, which was isolated from nematode worms in the Cape Verde islands. Verde1 is able to infect species like C. elegans, coating the outer surface, or cuticle, of the worms. The bacteria causes nematodes to stick together by their tail spikes in star-like formations. Hundreds of worms can become trapped in these aggregates and very few are able to escape. Those that become part of the star, die in the star.
The discovery of Verde1 (Leucobacter celer subsp. astrifaciens, which means “star maker”) is described in the International Journal of Systematic and Evolutionary Microbiology (available ahead of print here).
When the team behind the work grew Verde1-infected worms on a solid medium, they grew and behaved relatively normally. But, when they were put into liquid culture, the worms rapidly began to tangle together by their tails and form worm-stars.
“The worms have to be moving – dead worms don’t form stars,” explains Laura Clark, lead author on the paper. “If two worms pass each other, and their tails touch, they’ll stick to each other. The adhesion is really robust, so almost always if they touch, they’ll stick.”
Once three or four worms have bumped into each other in this way, the thrashing movements they make while trying to escape actually pull other worms into the star. And it’s this writhing that ultimately causes the worms to die – within 48 hours, all the worms in a star will have thrashed themselves to death.
“We think they die from the mechanical damage they cause themselves,” says Laura. “If you inject a dye that’s only taken up where the cuticle is damaged, you can see staining at the tails. And after the star has died, we see a proliferation of the bacteria, so we think they are using the worm carcasses for nutrients.”
The exact mechanism that the bacteria use to ensnare unsuspecting nematodes remains unclear. Worm-stars still form if dead bacteria are added to the culture, suggesting the role of a molecule that is always present on the outside of the bacteria, rather than a dynamic process. The worm’s surface biochemistry also appears to be important, as genetic mutants with different cuticles stick together differently – for example, across the whole body rather than just at the tail.
Elsewhere in nature, there are some species of predatory fungus that have also evolved to trap nematodes, in some cases using special lassoes to ensnare any worms that swim through them. Leucobacter astrifaciens is the first species of bacteria that has been shown to kill nematodes using the “trap-and-degrade” strategy of worm-stars – but the phenomenon may be more widespread than it first seems.
“Nematodes aren’t always studied in free-swimming states,” Laura explains. “But there are references in the literature to things like ‘rosettes’ or ‘medusa-heads’, which suggests that things like this have been observed before. For us, the interesting thing will be to figure out the mechanism of adhesion and what that can tell us about host–pathogen interactions.”
Clark, L., & Hodgkin, J. (2015). Leucobacter musarum subsp. musarum sp. nov., subsp. nov.; Leucobacter musarum subsp. japonicus subsp. nov., and Leucobacter celer subsp. astrifaciens subsp. nov., three novel nematopathogenic bacteria isolated from Caenorhabditis, with an emended descript International Journal of Systematic and Evolutionary Microbiology DOI: 10.1099/ijsem.0.000523