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 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. Their study is
published in the May 8 edition of PLoS
Pathogens.
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 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, senior author of the
study and an associate professor with the
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," he continued, "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 malaria parasite."
As part of the study, the researchers 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 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,"
Dimopoulos said.
Malaria kills more than 1 million people worldwide
each year, the majority of them children living
in Africa. Authors of the study are Yuemei Dong, Fabio
Manfredini and Dimopoulos. The research was
supported by the Johns Hopkins Malaria Research Institute
and the National Institute for Allergy
and Infectious Diseases.