May 10, 2009 — Bacteria in the gut of the Anopheles gambiae mosquito inhibit infection of the insect with Plasmodium falciparum,
the parasite that causes malaria in humans, according to researchers at
the Johns Hopkins Bloomberg School of Public Health. Scientists with
the Bloomberg School’s Malaria Research Institute found that removing
these bacteria, or microbial flora, with antibiotics made the
mosquitoes more susceptible to Plasmodium infection because of a lack of immune stimulation.
As part of the malaria transmission cycle, a mosquito acquires the
malaria-causing parasite when it feeds on blood from an infected
person. The parasite develops within the mosquito and can then be
transmitted to another human when the mosquito feeds again.
“Our study suggests that the microbial flora of mosquitoes is stimulating immune activity that protects the mosquito from Plasmodium
infection. The same immune factors that are needed to control the
mosquito’s infection from the microbes are also defending against the
malaria parasite Plasmodium,” said George Dimopoulos, PhD,
senior author of the study and associate professor with Johns Hopkins
Malaria Research Institute. “The interplay between bacteria and the
mosquito’s immune system may have significant implications for the
transmission of malaria in the field where mosquitoes may be exposed to
different types of bacteria in different regions. Theoretically, these
bacteria could be introduced to the mosquitoes to boost their immunity
to the malaria parasite and make them resistant and incapable of
spreading the disease. Our current research aims at identifying those
bacteria that trigger the strongest mosquito immune defense against the
As part of the study, the Johns Hopkins researcher treated
mosquitoes with antibiotics to kill the gut bacteria. Treated
mosquitoes were more susceptible to infection by Plasmodium
when feeding on infected blood compared to mosquitoes that were not
treated with antibiotics. To further verify the results, bacteria-free
mosquitoes were infected with bacteria to determine if they were less
susceptible to Plasmodium infection.
In addition, the researchers determined that mosquitoes infected
with bacteria died earlier than mosquitoes without bacteria when
infected with Plasmodium; 60 percent of the mosquitoes with
gut-bacteria died compared to 40 percent of those free of bacteria—even
with Plasmodium levels five times higher than those with bacteria.
“The malaria parasite must live in the mosquito for about two weeks
in order to complete its life cycle and be transmitted to a person. The
fact that these bacteria shorten the mosquito’s life span is additional
good news,” said Dimopoulos.
Malaria kills over one million people worldwide each year; the majority of deaths are among children living in Africa.
This research was supported by the Johns Hopkins Malaria Research
Institute and the National Institute for Allergy and Infectious
Diseases (NIAID), National Institutes of Health.