Fighting malaria with engineered symbiotic bacteria from vector mosquitoes
Sibao Wanga, Anil K. Ghosha, Nicholas Bongiob, Kevin A. Stebbingsb,1, David J. Lampeb, and Marcelo Jacobs-Lorenaa,2
aDepartment of Molecular Microbiology and Immunology, Malaria Research Institute, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21205; and bDepartment of Biological Sciences, Duquesne University, Pittsburgh, PA 15282
Edited by Nancy A. Moran, Yale University, West Haven, CT, and approved June 7, 2012 (received for review March 9, 2012)
The most vulnerable stages of Plasmodium development occur in the lumen of the mosquito midgut, a compartment shared with symbiotic bacteria. Here, we describe a strategy that uses symbiotic bacteria to deliver antimalaria effector molecules to the midgut lumen, thus rendering host mosquitoes refractory to malaria infec- tion. The Escherichia coli hemolysin A secretion system was used to promote the secretion of a variety of anti-Plasmodium effector proteins by Pantoea agglomerans, a common mosquito symbiotic bacterium. These engineered P. agglomerans strains inhibited de- velopment of the human malaria parasite Plasmodium falciparum and rodent malaria parasite Plasmodium berghei by up to 98%. Significantly, the proportion of mosquitoes carrying parasites (prev- alence) decreased by up to 84% for two of the effector molecules, scorpine, a potent antiplasmodial peptide and (EPIP)4, four copies of Plasmodium enolase–plasminogen interaction peptide that prevents plasminogen binding to the ookinete surface. We demonstrate the use of an engineered symbiotic bacterium to interfere with the de- velopment of P. falciparum in the mosquito. These findings provide the foundation for the use of genetically modified symbiotic bacte- ria as a powerful tool to combat malaria.
Anopheles gambiae | malaria control | paratransgenesis | transmission blocking
Malaria is one of the most lethal infectious diseases. Close to half of the population of the world is at risk, about 300–500 million contract the disease annually, and about 1.2 million people die of malaria every year (1). Continuous emergence of mosquito insecticide resistance and parasite drug resistance, combined with the lack of an effective malaria vaccine, severely limits our ability to counteract this intolerable burden (2, 3). New weapons to fight the disease are urgently needed.
Unlike the other two major infectious diseases (AIDS and tu- berculosis) that are transmitted directly from person to person, transmission of Plasmodium, the causative agent of malaria, strictly depends on the completion of a complex developmental cycle in vector mosquitoes (4). A mosquito may ingest on the order of 103 to 104 gametocytes from an infected human that quickly differen- tiate into male and female gametes that mate to produce zygotes, which, in turn, differentiate into ∼102 to 103 motile ookinetes. Ookinetes then migrate within the blood bolus until they reach the midgut epithelium, where they then traverse and differentiate into oocysts. Upon maturation, each oocyst releases thousands of sporozoites into the hemocoel, followed by invasion of the mos- quito salivary glands. The transmission cycle is completed when the infected mosquito bites the next vertebrate host and delivers some of the sporozoites with the saliva (5). Clearly, a severe bottleneck occurs during the mosquito midgut stages of parasite development: even in areas of high transmission, mosquitoes typically carry five or fewer oocysts (5). This bottleneck makes the mosquito midgut a prime target for intervention (6, 7).
One option to interfere with parasite transmission is to ge- netically modify mosquitoes for midgut expression of “effector genes” that inhibit parasite development. Past evidence suggests that this strategy works successfully in the laboratory (8–11). However, one unresolved challenge is how to drive transgenes
into wild mosquito populations. Various genetic drive mecha- nisms have been proposed to accomplish this goal (12, 13), but these are technically very challenging, and it is not clear in what time frame they will succeed. An important additional challenge faced by genetic drive approaches, in general, is that anopheline vectors frequently occur in the field as reproductively isolated populations, thus posing a barrier for gene flow from one pop- ulation to another.
An alternative strategy to deliver effector molecules is to en- gineer symbiotic bacteria from the mosquito midgut microbiome to produce the interfering proteins (paratransgenesis) (14). A key strategic consideration is that the mosquito microbiome (15, 16) resides in the same compartment where the most vulnerable stages of malaria parasite development occur (7). Moreover, bac- teria numbers increase dramatically (hundreds- to thousands-fold) after ingestion of a blood meal (15), and output of anti-Plasmodium effector molecules from engineered bacteria can be expected to increase proportionally.
Paratransgenesis has shown promise for the control of other insect-borne disease (14). Chagas disease caused by the parasitic protozoan Trypanosoma cruzi is transmitted by the triatomid bug, Rhodnius prolixus. In proof-of-concept experiments, an obligate commensal Gram-positive bacterium was genetically modified to secrete cecropin A, a peptide that kills T. cruzi, making the bug refractory to the parasite (14). Our previous study using the rodent malaria parasite Plasmodium berghei and the Anopheles stephensi vector mosquito suggested that recombinant Escher- ichia coli expressing on their surface either a dimer of the salivary gland and midgut peptide 1 (SM1) or a modified phospholipase reduced oocyst formation (17). Yoshida et al. (18) also showed that recombinant E. coli expressing a single-chain immunotoxin significantly reduces P. berghei oocyst density in A. stephensi mosquitoes. One limitation of these earlier experiments is that they used E. coli, an attenuated laboratory bacterium that sur- vives poorly in the mosquito gut (17). A second limitation was that the recombinant effector molecules either remained attached to the bacterial surface (17) or formed an insoluble inclusion body within the bacterial cells (18), thus preventing diffusion of the effector molecules to their parasite or mosquito midgut targets.
Here, we describe a substantially improved strategy to deliver effector molecules by engineering a natural symbiotic bacterium Pantoea agglomerans (previously known as Enterobacter agglom- erans) to secrete antimalaria proteins in the mosquito midgut. We found that the development of the human parasite Plasmodium
Author contributions: S.W., A.K.G., D.J.L., and M.J.-L. designed research; S.W., A.K.G., N.B., and K.A.S. performed research; N.B., K.A.S., and D.J.L. contributed new reagents/ analytic tools; S.W., A.K.G., and M.J.-L. analyzed data; and S.W., A.K.G., and M.J.-L. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1Present address: Neuroscience Program, University of Illinois at Urbana–Champaign, Urbana, IL 61801.
2To whom correspondence should be addressed. E-mail:
[email protected].
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1204158109/-/DCSupplemental.
12734–12739 | PNAS | July 31, 2012 | vol. 109 | no. 31
www.pnas.org/cgi/doi/10.1073/pnas.1204158109