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Parasite Kills Insect, Then Makes It Smelly and Unappetising

The caterpillar is all but invincible. It has bright warning colours that deter any birds which might want to eat it. It releases foul odours that deter hunters like beetle larvae, which rely more on scent. And it carries toxins that would make good on its threats. It’s a shame, then, that this caterpillar is dead. Its defences are the result of the creatures that killed it—an alliance between a parasitic worm and a glowing bacterium.

When the nematode worm Heterorhabditis bacteriophora burrows into an insect, it vomits out thousands of glowing bacteria, Photorhabdus luminescens. These release toxins that kill the insect and break its tissues into a nutritious soup, which the worms consume. The bacteria also make amino acids that the worms need to reproduce, and antibiotics that kill other microbes that might colonise the insect or decompose its corpse. (During the US Civil war, the same bacteria sometimes contaminated the wounds of soldiers, giving them an eerie blue shine while also protecting them from infections—they called it the “angel’s glow”.)

With the help of their bacterial allies, the worms grow within the dead insect, feeding, mating, and breeding. Eventually, their offspring burst out, suck up their own supply of killer bacteria, and head off to find their own hosts.

This takes around 20 days. During that time, the worms are exquisitely vulnerable: They will all die if a predator scavenges the insect host. And since the insect, being dead, is in no position to defend itself, the worms have to take over. They must protect the very body that they themselves killed. They must save it from being eaten from the outside so that they have enough time to eat it from the inside.

Again, their bacterial allies help. They produce toxins that can deter or kill ants, wasps, beetles, and other scavengers. And perhaps more importantly, they help to advertise these defences.

In 2011, Andy Fenton from the University of Liverpool found that the worms use pigments produced by the bacteria to paint their dead insects in warning colours, making them look as unpalatable as possible. As I wrote at the time, infected caterpillars start off as orange but soon take on a bright pink-red hue, which becomes more intense as the infections continue. This colour change was enough to deter robins, which hardly ever ate infected caterpillars but happily chomped down on healthy ones that had been dead for the same time.

Now, Fenton, together with Rebecca Jones and Michael Speed, have shown that the worms also release a pungent smell. They noticed it themselves: Even to their noses, infected insects stank in a way that uninfected individuals never did. Predatory ground beetles could tell the difference, too. When given a choice between the scents of infected and uninfected caterpillars, wafting out of separate jars, they almost always headed towards the uninfected smells.

It makes sense to produce warning smells as well as colours. The former can deter sight-oriented predators like birds, while the latter can put off smell-focused beetles, or foragers that operate at night. The colours also take several days to make, while the smells can be released soon after the infections begin, providing an earlier line of defence.

Indeed, there may be even earlier defences that the team haven’t discovered yet. Does the brief glow that the bacteria emit have a protective role? And since the bacteria produce light, toxins, off-putting odours, and warning colours, how do they prioritise between these different defences, given that each one takes energy to produce?

12 thoughts on “Parasite Kills Insect, Then Makes It Smelly and Unappetising

  1. I think it’s interesting, especially with the now obvious failure of our modern antibiotics, that these bacteria reduced the rate of infections in Civil War soldiers. Perhaps modern science can finally learn to work with nature, instead of trying to outsmart it, and ;use these bacteria as they are to fight infections. Our system of fighting infections has foolishly relied upon isolating single elements which the bacteria easily bypass. Nature, being far more resourceful than man, has always chosen the opposite approach with great success. I doubt we’ll do this, but one can always hope.

    1. Ok, you gotta understand this: Science always worked with nature. And medicine is no exception. Majority of major drugs came from nature. This is just a very nasty misconception medicine doesn’t take those things in consideration.

      Now, we can’t just use raw bacteria on infections, because bacteria can mutate and it can get nasty, that’s why drugs are extracted, to avoid this risk.

      1. Science has always tried to over-simplify nature. Extraction of single elements is done because the FDA is terrified of multiple-compounds. Garlic has still not been outsmarted by bacteria, while all of our synthesized, single compound antibiotics have. We haven’t worked with nature so much as we have tried to dumb it down to our level. Where has that got us? Just a few years ago, the WHO declared the age of antibiotics was over. This will eventually impact all areas of medicine as surgeries will be affected, and so on. Your fears were realized, but only because your reasoning was flawed.

    2. Most antibiotics originally came from microrganisms. What makes you think widespread use of these bacteria in some fashion won’t result in widespread bacterial resistance?

      1. Garlic is still effective against MRSA. I know this for a fact as I used it for such. The reason why? Because nature doesn’t isolate compounds, instead nature uses a plethora of defenses in it’s effective ones. Man isolates the compounds which means that the bacteria, which communicate with each other, can easily overcome it. But medicines like garlic uses literally hundreds of compounds, all entwined with a sulfur base. I thought I had adequately explained this earlier.

        1. So why was there such a drop in deaths from bacterial infection following the development of the first antibiotics? Garlic has been available for a long time.

          1. Because they were originally very successful. All the way up to the time when they weren’t. By the way, MRSA stands for Mycellum (sp?). It was first noticed in the late forties. Still called the hospital disease as that’s where most people get it. I didn’t. I got if from a cut on my finger and a handshake from someone who had gotten theirs from the hospital. This is all pretty basic knowledge. I’d hate to go into a conversation about how the scientific method is such a misused tool, using it to break everything down into it’s simplest elements isn’t always the best method. LIfe is far more complex than our perception of it. Bacteria, though they are denied the status of living creatures, act like very intelligent hive minds. They will constantly attack whatever we throw at them, and simple solutions are easier for them to overcome. That is why complex methods, such as garlic (there are many, but this one is one of my favorites), which primarily use a sulphur base are so effective. The more hurdles the better.

    3. Well we did make quite a lot of progress doing just that.

      These bacteria are an astonishingly good source of novel antimicrobials. Unfortunately there is a gap in funding schemes that makes it nearly impossible to translate novel antimicrobial “hits” into actual medicines. It’s not that biologists can’t find new antibiotics, it’s just that no one wants to pay for development. Shame really when there appears to be plenty of money for other things like bombs.

  2. Thanks, Ed, for highlighting research documenting such a remarkable symbiosis. Nematodes are some of the most abundant animals on the planet, and are entwined in tight symbioses with great allies: bacteria! Given that the killing of the host is accomplished by bacteria, the nematode is like a grim shepherd, shuttling the bacteria from host to host. Its not surprising then that the bacteria must quickly protect the cadaver (no different than a leopard hauling its fresh kill up into a tree, or a wasp burying its paralyzed spider in a subterranean gallery), but the eerie glow and scent generated by the bacteria–that’s other-worldly. Thanks also for the cool reference to “angel’s glow” among injured soldiers…

  3. Available retail for chafer biocontrol in the UK. I might have to get some to play with when it is available again next year.. Especially as the toxin is called mcf (makes caterpillars floppy).

  4. Great to see Photorhabdus getting attention. The most potent biological weapon nature has contrived as far as an insect is concerned. I actually work on human pathogenic forms called Photorhabdus asymbiotica. This is a misnomer really as they are also nematode symbionts. Sometimes the nematodes decide to burrow into humans and release the pathogen. The pathogen is actually well adapted to causing a horrible infection we’ve called photorhabdosis. We are about to publish a very detailed paper on how these insect pathogens have made the jump to humans in PLOS ONE. It uses all the same toxins, yes even Mcf, it just changes what molecules it uses to metabolise. The Angel glow reference is just an anecdote by the way. If the soldiers got Photorhabdosis it would likely have killed them. Photorhabdus is generically closely related to the plague bacterium yersinia.

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