The life history of Ampulex compressa, the emerald cockroach wasp, is likely to make you squirm. A female wasp stings a cockroach—momentarily paralyzing its front legs—and drags it by its antennae to her burrow, where she glues a single egg to the roach’s leg. Once the larva hatches, it feeds carefully on the internal organs of the still-living insect, keeping it alive until the larva is ready to pupate. At that point, the larva spins a cocoon inside the roach's exoskeleton and finally emerges as an adult, leaving the carcass of the now-dead cockroach behind.

The cockroach serves as both the restaurant and the meal for the wasp larva, but roaches are a notoriously filthy species that harbor and transmit some pretty nasty pathogens like Salmonella, E. coli, and the eggs of various roundworms. How do the larvae protect themselves from these pathogens while dining on the cockroach’s internal delicacies?

In this week’s issue of PNAS, a group of researchers report that emerald cockroach wasp larvae produce an antimicrobial secretion that helps sanitize the insides of their cockroach hosts.

By peering into holes in cockroaches’ abdomens (which the ever-resourceful scientists covered with coverslips), the researchers observed the wasp larvae depositing small drops of a clear oral secretion into the roaches’ body cavities. They found this secretion had at least two compounds—(R)-(-)-mellein and micromolide—with known antimicrobial properties. (R)-(-)-mellein is known to inhibit the processing of some proteins related to hepatitis C, and micromolide is known to work against the bacteria responsible for tuberculosis.

The scientists extracted these compounds from the larvae’s secretions and tested them against two bacteria commonly harbored by cockroaches: Serratia marcescens, a Gram-negative bacterium, and Staphylococcus hyicus, a Gram-positive one. Together, the two compounds were very effective at inhibiting the growth of each of these pathogens, reducing bacterial growth by more than 50 percent. Further testing showed that (R)-(-)-mellein was more effective at keeping S. marcescens at bay, and micromolide was the better inhibitor of S. hyicus.

(R)-(-)-mellein and micromolide aren’t known to occur together anywhere else in the natural world, which suggests this combination may be an evolutionary adaptation unique to the emerald cockroach wasp. Because of their rather bizarre life history, these wasps likely encounter a huge number of pathogens during their lifetime. Having a broad range of antimicrobial compounds available to them may be extremely advantageous.

Thanks to this new knowledge about their antimicrobial properties, these compounds could contribute to future medicines and drug therapies. The researchers also acknowledge there are likely other unidentified compounds inside the wasps’ secretions that may help us fight disease in the future.

PNAS, 2013. DOI: 10.1073/pnas.1213384110 (About DOIs).