Formosan subterranean termites are among the most socially sophisticated of animals. Living in underground colonies millions strong, these insects share the work of foraging and defence while jointly benefiting from the fruits of their labours. However, with the advantages of social life come certain challenges. For a start, there is the problem of disease risk in a colony comprised mainly of siblings. And then, what do so many termites do with all of their waste? A recent report published in the Proceedings of the Royal Society, Series B, describes a novel integration of sanitation and public health that allows termites to tackle these problems together.
Colonies of Formosan termites are like vast underground cities. Tunnels that termites travel in search of food extend for hundreds of feet through the soil. These emerge into hollowed trees, buildings and homes, where termites construct dwellings called ‘carton nests’ composed of chewed wood, saliva and termite faeces. Unsurprisingly, these nests are not only comfortable homes for termites. A group of scientists led by Thomas Chouvenc at the University of Florida in Fort Lauderdale, USA, discovered that nests also support the growth of bacteria and fungi. However, pathogenic fungi are notably absent from the carton nests. This is a good thing for termites, of course, but how are the nasty fungi kept at bay?
When the researchers tested whether the nest itself was antifungal, this proved not to be the case. Fungal pathogens inoculated into a growth medium made from termite cartons – appealingly called termite faecal agar – grew rapidly and reached high densities. By contrast, when the faecal agar was co-innoculated with a particular bacterial species isolated from the carton nests, fungal growth was arrested. Looking further, the team found that this same bacterial species, a type of filamentous bacterium called a streptomycete, secreted compounds that directly inhibited fungal growth.
Several studies over the past decade have published similar results in a broad range of insect species: an insect is squashed, a streptomycete is isolated from the corpse and this bacterium is found to retard growth of a pathogen that would attack this insect in an in vitro assay. The story usually ends here, with the inference that the streptomycete derived from the insect somehow aids the species in disease control. What is particularly interesting about the present study is that Chouvenc and colleagues could put this inference directly to the test under semi-natural conditions. They built mini termite nests and monitored the survival of colonies through time. Cleverly, by using nest material composed initially of sterile soil, the researchers could directly manipulate termite exposure to either friendly or harmful microbes. As anticipated, colonies exposed to pathogens died most rapidly. But when these colonies were also exposed to streptomycetes, the termites survived as well as they did in sterile soil. This result thus provided clear evidence that streptomycetes help protect Formosan termites against lethal infection, thereby providing them with a form of external immunity.
Less clear from this work is how this relationship evolved. Do termites specifically attract streptomycetes using faecal bait, perhaps farming them to exploit their antifungal secretions? Or do termites incidentally benefit from the fact that streptomycetes prolifically secrete antimicrobials to protect their own food, pre-digested wood pulp (termite faeces) in this case, from fungal competitors? Studies to distinguish these intriguing possibilities are in progress.