Entomologist Cameron Currie of the University of Wisconsin and his colleagues discovered the antibiotic bacteria in crescent-shaped pits on the exoskeletons of two species of Panamanian ants, Cyphomyrex longiscapus and C. muelleri, after scanning them with an electron microscope. The bacteria--of the Pseudonocarida genus--bloom on the individual face plates and other exterior parts of the ant, allowing it to rub the antiparasitic agent on its fungi crop. The ant also nurtures the microbe by secreting nutrients from special exocrine glands connected to the shallow pits.
"Every ant species [that we have examined] has different, highly modified structures to support different types of bacteria," Currie observes. "This indicates that the ants have rapidly adapted to maintain the bacteria. It also indicates that the coevolution between the bacteria and the ants, as well as the fungus and parasites, has been occurring since very early on, apparently for tens of millions of years."
In fact, more than 200 species of ants display this complex symbiosis, according to Currie. "It now appears that the fungus-growing ants are more modified for culturing their mutualistic bacteria than for their mutualistic fungi," Currie notes.
The unexpected finding also bears promise for human agriculture and medicine: the ants have been able to avoid promoting resistance for as long as 50 million years. "I think it has to do with the ants having several mechanisms to suppress the parasite," Currie says. "In addition to the bacteria, the ants have specialized behaviors that involve removing the parasite from the fungus garden."
The Attine ants join a short list of other insects, animals and plants known to harbor beneficial microbes. Many more such species may remain to be discovered, however. "For me, it shows us how little we know about the natural systems and microbes in nature," Currie adds. "Fungus-growing ants are very well studied, yet this morphological characteristic went unnoticed until now. What other organisms might be taking advantage of this type of association? What don't we know about other systems that are not as closely studied as those of ants?"