(Note: This post is not for the squeamish)
All kinds of stuff lives in our guts. It’s been estimated that there are about ten times as many bacteria cells (microflora) in our body as human cells. Most of them are totally harmless, and indeed, they are often beneficial. But as the authors of a paper last week in Science pointed out, bacteria aren’t the only things living down there, and the beasties that live in our gut don’t always play nice, at least, not with us:
The inhabitants of the mammalian gut are not always relatively benign commensal bacteria but may also include larger and more parasitic organisms, such as worms and protozoa. At some level, all these organisms are capable of interacting with each other. We found that successful establishment of the chronically infecting parasitic nematode Trichuris muris in the large intestine of mice is dependent on microflora and coincident with modulation of the host immune response.
Trichuris is a genus of helminth worms that live in the large intestines of mammals. They are wildly successful; over 1 billion (yes, Billion) people are estimated to be infected with T. trichiuria every year. Infection starts with ingesting Trichuris eggs with your food. The eggs are able to survive the harsh environment of your stomach and make their way down to the large intestine, where they hatch and begin feeding, mating, and laying new eggs. When you defecate (we just can’t get away from poo on this blog evidently), the eggs are free to contaminate other food. If you read between the lines correctly, this means that at the beginning of the life-cycle, you must have eaten food contaminated with fecal matter (there’s a reason that this parasite doesn’t do well in regions of the world with good sanitation). This is not nearly as gross as the life-cycle of hook-worms, which can burrow into your foot, swim through your blood-stream to your lungs, then wriggle their way into your mouth through your trachea to be swallowed while you sleep – but I digress.
These researchers wondered what would trigger these ingested Trichuris eggs to hatch. If the worms hatch before they get ingested, they’ll die before they get there, and if they hatch too late, they’ll be expelled before they can establish themselves in the gut. So what is there a lot of in the location they want to hatch, that could provide a signal? Bacteria! When the researchers dumped all kinds of different bacteria on the worm eggs, they hatched. In addition, if they treated mice with high-doses of antibiotics to clear out their guts, and then fed them worm eggs, the worms didn’t hatch. Crucially, dumping bacteria on the eggs only worked if they were at body temperature (37 degrees C); at room temperature, it didn’t work. This makes sense from the worm’s perspective too – there are bacteria everywhere (especially in the poo they’re originally expelled in), but the only place there is likely to be lots of bacteria at 37 degrees is in a mammalian gut.
This doesn’t mean that we should go treating everyone in the 3rd world with antibiotics to prevent worm infections – the normal bacterial flora are too important. But it does raise some potential new lines of preventative therapy. While the exact nature of the interactions between worm egg and bacteria were not complete worked out, they did find that interactions with some types of bacteria were dependent upon a certain class of adhesion molecules, and that these interactions could be blocked by mannose. More understanding of how these different inhabitants of our intestines communicate at the molecular level will doubtless lead to great therapies, but as most of these infections happen in the third world, those therapies will have to be cheap if they’re to do any good.