Attacking Plasmodium falciparum by modulating the immune system of its natural mosquito vector is expected to provide an effective alternative means of curbing malaria, according to George Dimopoulos and his team at the Johns Hopkins Bloomberg School of Public Health and Malaria Research Institute in Baltimore, Md.  "Similarly to humans, the mosquito intestine harbors an immune-stimulating natural microbiota, which, in the insect, enhances its ability to defend against infection from the malaria parasites," he says. Dimopoulos was one of several speakers who described novel approaches to reducing the worldwide malaria burden during a Johns Hopkins Malaria Research Institute (JHMRI) Web summit held this past November.

"Knowing which bacteria best enhance the mosquito's immunity against infection could have significant implications for the transmission of malaria in the field where bacterial exposure varies greatly from niche to niche," Dimopoulos says. "Thus we're using comprehensive functional genomics to identify the mosquito's core gut microbiome and the molecular interplay between its bacterial residents and the Plasmodium parasites."

When members of the JHMRI team examined the microbial load and species composition of laboratory-reared Anopheles gambiae mosquitoes, they found great variation in those microorganisms, even between mosquitoes in the same cage. "However, as in previous studies, the majority of isolated bacteria were gram-negative," Dimopoulos, says. Sequence analyses from morphologically distinct bacterial colonies consistently showed the dominant organisms to be Enterobacter asburie, Microbacterium sp., Sphingomonas sp., Serratia sp., and Chryseobacterium meningosepticum.

The JHMRI group is also investigating the genes responsible for Anopheles anti-Plasmodium immune responses and has specifically addressed the role of Toll and Imd immune signaling pathways. For instance, the gene caspar, which encodes a regulator of the Imd pathway, controls mosquito resistance to P. falciparum. Importantly, caspar silencing results in resistance to the human Plasmodium parasite in three divergent and major anopheline malaria vector species in South America, Asia, and Africa. This immune response enhancement does not appear to significantly compromise the mosquito's fitness. Further, the team produced two lines of viable, Imd pathway-enhanced mosquitoes and is working on a third type which combines the immune potency of both its predecessors. Details of this research appear in the March and May 2009 PLoS Pathogens.

It is one thing to produce transgenic, malaria-resistant mosquitoes but quite another for them to spread their genes to wild-type insects, says Marcelo Jacobs-Lorena, also at the Bloomberg School of Public Health. Because mosquito fitness, including a robust reproductive capacity, is critical, he and his collaborators are taking another tack-making the mosquito's natural bacteria lethal for Plasmodium and feeding those altered bacteria to wild mosquitoes on sugarimpregnated cotton balls. It is easier to grow large numbers of genetically modified bacteria than large numbers of transgenic mosquitoes, Jacobs- Lorena says.

"Testing for malaria remains a problem, especially in areas where there are taboos against taking blood and suspicion of the few health care workers we have available," says Web summit participant Sungano Mharakurwa, who is Scientific Director of the Malaria Institute at Macha (MIAM) in Zambia. Thus, he and colleagues are developing tests that detect P. falciparum antigens and nucleic acid in saliva as accurately as in blood. "We're also collaborating with scientists at New York University in the U.S. to turn our saliva benchtop assay into a simple, noninvasive, point-ofcare diagnostic tool usable at the grassroots level." This, says Mharakurwa, will enable trained lay people to find and treat human carriers during the dry season, reducing sources of infected blood for mosquitoes to spread when the rains begin (see Microbe, January 2008, p. 9).

Meanwhile, Gregory Glass, also at the MIAM site in Zambia, is using geographic information system mapping to develop a spatial relationship between disease parameters and local geographical features and to track that relationship as it changes with time. This information gives local researchers such as pediatrician Phil Thuma, Managing Director of MIAM, detailed information about the dynamics of malaria risk in the area. Concerted antimalaria efforts in the Macha region have drastically reduced the number of cases over the past five years, Thuma says.

The malaria parasite's apicoplast organelle houses unique metabolic pathways and is likely to provide novel therapeutic targets, according to Sean Prigge of JHMRI. "The main point of the summit [was] highlighting basic research like mine, which is the mission of the JHMRI, and showing how it can augment existing antimalarial strategies," he says.

More information about the JHMRI Web summit is available at http://malaria.jhsph.edu/malariasummit2009.

Marcia Stone