In the eternal war between hosts and pathogens, both sides have to come up with furtive ploys with which to beat the other. The same is true of the fungal parasite microsporidia, which is known to infect large numbers of the population, and cause diarrhea. Yet how it infects the cells has long been unknown.
It turns out that rather than infecting one cell, exiting and then moving onto the next, it has a stealthy trick up its non-existent sleeve. After breaking into the first cells, the parasites will then force the host to fuse with their neighboring cell, allowing them to move into the next uninfected cell. While this sort of behavior has been observed before in bacteria and viruses, this is the first time that parasitic eukaryotic organisms have been found to infect hosts in this way.
“It's like microsporidia have figured out that, like soldiers fighting in an urban warfare, it's easier and safer to go from house to house by entering adjacent houses through a common wall, rather than by going through the front door of each house,” explains Emily Troemel, who led the study published in Nature Microbiology. “This is the first time we've seen this mode of infection by a eukaryotic pathogen, which is a single-celled organism with a distinct nucleus containing genetic material.”
The researchers discovered this fascinating mode of infection by observing how the parasite invades the transparent roundworm Caenorhabditis elegans, an organism commonly used in biological research. By tagging the RNA in the microsporidia with red fluorescent dye, and then labeling the cell membrane of the worm's gut in green, it allowed them to watch how the parasite infects and spreads within the organisms cells. And what they found surprised them.
The flatworm C. elegans is used in research because despite being about as primitive a multicelled organism as you can get – comprised of just 959 cells, all of which have been mapped – it none the less shares many characteristic with human biology. The roundworm allows scientists to precisely follow the fate of individual cells, which is exactly what the team did when infecting the worm's gut, which is made up of just 20 cells, with microsporidia. They found that after infecting the first, within two days the parasite had managed to fuse all 20 together, replicating itself within them all.
They then repeated the experiment but instead infected a single muscle cell with the parasite, and watched how it did exactly the same thing, invading the first, and then fusing it to its neighbor before moving over. The fact that they found the same method employed in two different tissue types implies that this mode of infection is fairly standard for the fungus, and therefore is likely to occur in our own guts, too.