Who said DoTW! could only do human diseases? Why are you so special huh? Insects get sick too and someone has to care. I care. Also, by showing some sympathy about insect diseases maybe I will survive the invasion and subsequent enslavement of the human race by our most welcome and superior Insect Lords. I pledge to humbly serve thee.
So why an insect pathogen?
Well, it’s fun to be different I guess. Also the bacteria I’m going to discuss, Photorhabdus luminescens, is fun to say. They also happen to be really interesting for a bunch of reasons:
- Can’t cause disease in insects without help from a friend
- A classic example of symbiosis
- Referred to as the ‘Angel Glow’, or something similar, by ‘the wounded’ anytime before World War II
See I told you they were interesting.
Photorhabdus: the bane of our insect overlords
We haven’t spoken much about life-cycles on the previous posts because generally we have been dealing in human pathogens which, for the most part, struggle to exist and cause disease in anything but humans. This life cycle is very simple human-human-human etc. Other pathogens such as the causative agent for malaria are spread by mosquitoes so the life cycle is more complicated. For Photorhabdus we have a very intriguing life cycle indeed.
Photorhabdus are a group of bacterial insect pathogens but other than that are not particularly fussy. This fact alone is quite interesting. Most pathogens have specialised to infect one or a small group related animals. There are few pathogens that can infect, say, all mammals equally. The other thing unusual about how the infection takes place is that the bacteria cant do it on their own, they require the assistance of a ‘taxi’ nematode (a family of multi-cellular, micro-organisms, look a lot like a microscopic worms in most cases) called Heterorhabditis. The nematode enters insects and proceeds to regurgitate the bacteria, which live in its intestine, into the insect. The bacteria thrive in the insect and quickly multiply and grow. During this time the nematode begins feeding on the bacteria whilst also growing and multiplying. Once the insect has died and its cadaver is entirely hollowed out the nematodes stop feeding on the bacteria and move off to another host, where the process starts again.
Why would the bacteria and worm have developed this relationship together? Surely it would have been easier for the worm to evolve the ability to feed on the insect directly, and wouldn’t it make more sense if the bacteria didn’t grow just to be eaten by the worm again? This is where the classical model of symbiosis I mentioned comes in.
Symbiosis is observed when two dissimilar organisms actively co-exist, most often to the benefit of both organisms. When both organisms benefit from co-existing together it can also be called mutualism. In this case the bacteria in the environment are not guaranteed nutrients on which to grow, neither are nematodes. By working together they can each ensure their own survival as they now ‘rely’ on each other. In fact the nematode has evolved with such a preference for Photorhabdus feeding that now it absolutely requires it. This produces an interesting problem for the nematode, the insect is harbouring other bacteria which are potentially toxic to it. How can it ensure it only eats Photorhabdus? Lucky for the nematode the bacteria are on the case.
Upon being regurgitated the bacteria start to produce a range of anti-microbial factors that inhibit the growth of other bacteria. These include a family of bacteria specific toxins called pyocins. But how can the bacteria make sure they get eaten by Heterorhabditis and not some other worm? Again the bacteria have ‘thought’ about this. They also produce a number of molecules that kill nematodes, but not Heterorhabditis.
So the bacteria protect the worm from eating the wrong bacteria and also ensure it can only be eaten by the right worm. Pretty smart huh?
Well they have one more trick, and it’s awesome. They glow.
Why? We’re not actually sure but one of the strongest hypotheses around currently is that once the bacteria are present in very large numbers, like when the current insect corpse is running out of flesh, the glow of the large number of bacteria together attract other insects to the corpse, making the transition from the ‘spent’ host to a new host easier. Oh, the bacteria also produces a complex molecule capable of killing insects that attempt to scavenge the corpse, allowing super easy transition into a new host.
These are very cool bacteria.
There’s always a human angle…
Okay, so maybe I wasn’t being entirely honest when I made a big deal about the expectations of these columns being about human diseases only. Turns out the bacteria have an interesting human angle that in fact was the first thing that attracted me to them. Whilst it is recognised as a human pathogen it is typically asymptomatic and is extremely rare with only 14 reported cases in medical literature. So it’s not a human disease that interests me.
What I find interesting is that Photorhabdus has been suggested as the cause of a mysterious phenomenon called ‘Angels glow’. If you were looking for a set of conditions in which Photorhabdus, an insect pathogen that normally lives in the soil, might be able to grow in a human you would need a few things. Open wounds, where soil etc might be able to access the body’s rich nutrient supply, a lot of exposure to soil, poor nutrition, inadequate medical treatment would all be required, and where would you find these conditions met? Trench warfare.
Some soldiers reported that their wounds glowed and reports taken at the time suggest glowing wound rarely got infected and the patients had a much better prognosis. The glow was seen as a gift from the angels thus ‘Angels glow’. It is widely believed that the glow was Photorhabdusluminescens growing in the wounds of soldiers. The wounds didn’t become infected due to all the nasty stuff spewed out by the bacteria that killed off potential disease causing pathogens allowing the wounds to heal normally. It is expected the bacteria would eventually be removed by the immune system but persist long enough to prevent the growth of any nasties. Whilst it is convenient to attribute the phenomenon known as the ‘Angel glow’ to Photorhabdus we don’t have any remaining data to link the two definitively, what we have however does put it forward as the leading explanation.
One more thing about this bacteria and it’s human connection. What does the bacteria and nematode feed on? Insects. Where are insects a problem? Agriculture. This particular pairing of Heterorhabditis and Photorhabdus is currently in use as a biopesticide in a number of countries (including Australia). As it is not a manufactured chemical and there is currently a demand for organic produce, it is expected that the biopesticide industry, where natural predators for insect pests are used instead of standard herbicides and insecticides, will flourish due to its promotion as a natural alternative.
Gerrad J. et al., (2003). “Photorhabdus Species: Bioluminescent Bacteria as Emerging Human Pathogens?” Emerging Infectious Disease V9 (2)
Waterfield N. et al., (2009). “Photorhabdus and a Host of Hosts” The Annual Review of Microbiology V63