|Steinernema carpocapsea larva in its free-living phase (left), and its infective/parasitic phase (right)
Photos from Fig 1 of the paper
For the average insect, a lethal dose of X. nematophilus consists of about 3500 bacterial cells. But, each S. carpocapsae only carries 20—200 cells of X. nematophila - well below the lethal dose. The fact that a single worm is enough to kill an insect host with so few bacterial cells means that S. carpocapsae isn't just relying on the bacteria to do all the dirty work.
When a newly spawned S. carprocapsae crawls out of an insect carcass into the outside world, they look like just another nondescript soil nematode. They do not feed during that stage, so their mouth and guts are sealed shut. But when a S. carpocapsae larva encounters a suitable host, its body starts changing - its head swells up, its mouth opens, and its gut expands (see the photos above). It's like going through puberty, except instead of getting acne on your face or have hair sprout from certain places, or your voice changes, S. caprocapsae turns from a seemingly innocuous worm into a lean, mean parasitic killer.
But aside from such physical changes, these infective larvae also start spewing out a complex cocktail of proteins. When researchers isolated and examined this mixture more closely, they found that it was made up of 472 different proteins - many of them are proteases, which are digestive enzymes that breaks down proteins and cellular structures. There are also some peptide toxins similar to those found in other parasitic nematodes, but the functions of the vast majority of those molecule are unknown. And it turns out this cocktail can be quite toxic for insects. Fruitflies that are injected with S. carpocapsea toxins die within two to six hours, and it proved equally deadly for silkworms. Waxwmoth larvae fared a little better - while the toxins left them paralysed, they were able to recover after 24 hours, though a bit battered and bruised from the experience.
As deadly that might seem, based on the outcome of the lab experiments, it would take 20 parasite larvae about 24 hours to produce enough toxin to kill a fruitfly - which is a far cry from what goes on in the wild where a single S. carpocapsae can take down insects larger than fruitflies within two to three days. This nematode cocktail also expires pretty quickly, and completely loses its killing power after 54 hours.
However, we have to keep in mind that the proteins S. carpocapsea produces are not acting alone. Despite the parasite's toxic arsenal, its symbiotic bacteria still plays a very important role in killing the insect host. Also of the hundred of proteins that that S. carpocapsea secretes, not all of them contribute to the insect-killing process through sheer toxicity, some might work in conjunction with some of the bacteria's own toxins to boost their lethality. Some might be running interferences that suppress the host's immune system, which is a distinct possibility given their similarity to the peptide toxin secreted by other parasitic nematodes.
Understanding how all these proteins work, and how they function with S. carpocapsae's bacterial symbionts would require further investigation. With its arsenal of toxins and deadly bacterial symbiont, S. carpocapsae is the stuff of nightmares for insects in the undergrowth. But it may also give us insight into how parasitic nematodes overcome or subvert their host's defences, and how animal-microbe symbioses function in their respective environments.
Lu, D., Macchietto, M., Chang, D., Barros, M. M., Baldwin, J., Mortazavi, A., & Dillman, A. R. (2017). Activated entomopathogenic nematode infective juveniles release lethal venom proteins. PLoS Pathogens, 13(4): e1006302.