By Dean Haycock

Special To BioWorld Today

Bugs use germs to make us sick. Why can't we use germs to neutralize bugs?

That is just what scientists are doing. Using the transmission of Chagas' disease as a model system, researchers from the Yale University School of Medicine, in New Haven, Conn., the Centers for Disease Control and Prevention (CDC), in Atlanta, and Rockefeller University, in New York, are developing a method to kill disease-causing microbes living in insects. Their weapon is a genetically engineered version of a bacterium that normally lives in the insect's gut.

The insect in this model system is a bloodsucker that belongs to a group of insects called reduviid bugs. They look somewhat like cockroaches with probing mouth parts. Their common name, "kissing bug," alludes to their habit of biting people on the face. This unwanted attention too often results in transmission of Trypanosoma cruzi, a protozoan that causes Chagas' disease.

Also called American trypanosomiasis, Chagas' is endemic in nearly all rural areas of Central and South America. The course of the disease can be mild or can end in death, especially in children. There is no drug cure. Controlling the insect carrier is the best hope health workers have. In hopes of achieving that goal, researchers genetically transformed bacteria that live symbiotically in the gut of the kissing bug to produce a peptide called cecropin A. Cecropin A kills the trypanosome parasite but spares both the transformed bacteria that produces it and the kissing bug itself.

The description of the first successful use of genetically transformed symbiotic bacteria to counter disease transmission by insects is described in "Prevention of insect-borne disease: an approach using transgenic symbiotic bacteria." The paper appears in the April issue of the Proceedings of the National Academy of Science (PNAS).

"This is a very attractive approach to controlling disease. It has a couple of important features," Angray Kang, of the Scripps Institute, in La Jolla, Calif., said when asked to comment on the paper.

The approach, for instance, avoids difficulties associated with attempts to create transformed insects. "We can move along a lot faster with the symbiont approach. We are not sitting around twiddling our thumbs, waiting for transgenic insects to arrive. We can actually start testing ideas," Kang said.

The technique eliminated or significantly reduced the disease-causing protozoan in insects whose normal gut bacteria had been replaced with genetically engineered bacteria, according to first author Ravi Durvasula, an instructor at Yale, and his colleagues.

Maintaining Nature's Balance

The fact that the treatment does not kill the kissing bug may not mean much to many people, but from an ecological standpoint, it is significant. Insects fill ecological niches and their complete elimination can have consequences that ultimately affect other species. By specifically eliminating the disease-causing microbe in the insect rather than the insect itself, both humans and the general ecology may benefit.

The use of transgenic symbiotic bacteria to control the spread of disease may one day bolster efforts that now depend on use of insecticides (which insects can develop resistance to) and of sterile males (which, entomologists are discovering, may be shunned by savvy females).

To disseminate the transformed bacteria throughout an insect population, the scientists hope to exploit a unique behavior of the kissing bug. Although the insect that transmits Chagas' disease is a bloodsucker, it does not directly transmit the disease while sucking blood. It does so by depositing a fecal droplet containing the trypanosome at the time of the blood meal. The microbes enter humans either through small cuts in the skin or through mucus membranes.

The kissing bug often probes fecal droplets left by other insects, a behavior called coprophagy. Since both the transgenic symbiotic bacteria and the protozoan are present in the droplet, the engineered bacteria could possibly be spread throughout a colony via this behavior. This method of spreading the transformed bacteria could work for other insects that demonstrate coprophagic behavior.

This is significant, Kang told BioWorld Today, because they have a definite method for spreading the [transformed] symbionts."

The researchers now are trying to identify the symbiotic bacteria present in other insects that transmit Chagas' disease with the aim of genetically manipulating them to help eliminate the trypanosome.

Staying Ahead Of The Resistance Factor

"We are also working on cloning the genes for other peptides. We are concerned that, in the long run, if we express only one peptide, the trypanosome could become resistant to it quickly. We would like to keep several options open down the line," Durvasula told BioWorld Today.

Kang cited another advantage of the technique described in the PNAS paper: "Because it is so easy to engineer symbionts, if you get resistance, you can have a whole panel of factors ready to go." The approach of using engineered symbiotic bacteria to control insect-borne Chagas' disease could be applied to many other diseases of plants, animals and humans.

"We are trying to develop this as a system that may be generic. We would look for insects that contain symbiotic bacteria that are amenable to transformation," Durvasula said. Ticks and fleas are two good candidates, and already basic research has begun toward eventually applying the technique to the control of plant diseases spread by aphids and plant-hopper insects and toward the control of African trypanosomiasis.

In collaboration with an overseas partner of the CDC, the Medical Entomology Research Training Unit, in Guatemala, the researchers have begun simulated field studies as a step toward one day moving the system from the lab to the field.

"We are recreating field conditions in containers to see if we can co-infect insects with our transformed bacteria," Durvasula said.

Durvasula sees potential commercial applications of the technique in the fields of agriculture, aquaculture and veterinary medicine as well as human medicine. *