Allowing mosquitos to feed on engineered strains of the symbiotic bacteria that naturally live in their midguts may provide the answer to preventing the malarial parasite Plasmodium from completing the relevant stages of its life cycle in the airborne host and being transmitted to humans, researchers claim. A team at the Johns Hopkins Bloomberg School of Public Health’s Malaria Research Institute has generated engineered strains of a common symbiotic bacterium Pantoea agglomerans that resides in the midgut of the anopheles mosquito. The bacterium is modified to secrete proteins that directly block development and survival of the Plasmodial ookinetes and oocysts developing in the midgut, which would normally give rise to the sporozoites that are transmitted into humans through the mosquito’s saliva.
Marcelo Jacobs-Lorenam Ph.D., Sibao Wantg, Ph.D., and colleagues found that anopheles mosquitoes given these modified P. agglomerans bacteria and subsequently allowed to feed on blood meals containing either P. falciparum or P. beghei, developed up to 98% fewer plasmodial oocysts. The authors report their findings in PNAS in a paper titled “Fighting malaria with engineered symbiotic bacteria from vector mosquitoes.”
Human Plasmodium parasites have a complex life cycle that needs both the anopheles mosquito and human hosts for completion. In theory, blocking one part of the parasite’s life cycle in the mosquito should prevent the transmission of viable parasite to humans. Previous work has attempted to engineer the mosquito hosts such that they secrete antiplasmodium factors that inhibit development of the parasitic ookinetes and oocysts in the midgut. However, the prospect of getting transgenes into wild mosquito populations is proving particularly challenging.
The Johns Hopkins researchers thus looked instead to administering the mosquitoes wtih engineered symbiotic bacteria that would effectively do the same job, that is, secrete effector molecules into the midgut that block the Plasmodium life cycle at the stage of ookinete and oocyst development, and thus prevent the production of sporozoites that are transmitted to humans. This approach, they point out, is particularly attractive because the number of bacteria in the mosquito midgut increases dramatically after ingestion of a blood meal, and thus should increase the output of effector molecules produced by recombinant bacteria.
The team’s tests used engineered P. agglomerans strains that expressed five different antiplasmodial effector molecules, each of which acts by a different mechanism to prevent development and migration of ookinetes, or maturation into oocysts. The bacteria were administered by allowing the mosquito to feed on a sugar solution infected with the relevant engineered P. agglomerans strain.
Significantly, the results showed that there was up to an 84% drop in the proportion of parasite-carrying mosquitoes (both human P. falciparum and rodent P. berghei strains) following administration of P. agglomerans engineered to secrete either the antiplasmodial proteins scorpine, or Plasmodium enolase–plasminogen interaction peptide (EPIP)4. Moreover, the approach was effective in different species of anopheles that feed on humans, including A. Gambiae (an African mosquito) and A. Stephensi (an Asian mosquito).
Given the feasibility of using engineered bacteria as a strategy for controlling malaria, one of the next stages will be to determine how to introduce the bacteria into mosquitos in the field. One possibility would be the establishment of baiting stations around the villages where malaria is prevalent to allow the mosquitoes simply to feed from cotton balls soaked with sugar and the engineered bacteria.
“These findings provide the foundation for the use of genetically modified symbiotic bacteria as a powerful tool to combat malaria,” the authors conclude. “It is important to note that paratransgenesis is compatible with current mosquito control tools (insecticides, population suppression) and even with genetically modified mosquitoes.... Should the technical barriers for transgene introgression into mosquito populations be solved, we envision the concomitant application of both strategies (genetically modified mosquitoes and paratransgenesis), because they can complement each other.”