Over the past decade, promising reports have suggested that Wolbachia infection can inhibit pathogens in mosquitoes including dengue, chikungunya, and yellow fever viruses, and malaria parasites. Additionally, some strains of Wolbachia can shorten insect lifespan, and shorter-lived vectors mean less opportunities for transmission of pathogens. However, in many cases, infecting mosquitoes with Wolbachia has proven more difficult than anticipated. Despite high prevalence in other insects, including the mosquito species from which Wolbachia was first isolated, Culex pipiens, difficulties in infecting Anopheles mosquitoes led researchers to presume this genus was incapable of harboring Wolbachia. While studies, have now shown that this is not the case, including work from the Rasgon lab, infection of Anopheles with Wolbachia results in mosquitoes with extreme fitness costs, such as high mortality after blood-feeding, making the idea of stably infecting wild mosquitoes with the bacteria unlikely to be successful.
Why is Wolbachia easily transmitted in some mosquitoes but not in others? Hughes and CIDD colleagues decided to explore whether characteristics of the Anopheles mosquito microbiome may be inhibiting the ability of Wolbachia infections to take and spread. Knowing that competitive interactions between different types of bacteria can lead to competitive exclusion of one bacteria in the presence of another, the researchers treated mosquitoes with a cocktail of antibiotics before infecting with Wolbachia and compared the stability of Wolbachia-infection in these mosquitoes to mosquitoes infected without prior use of antibiotics. Wolbachia was more likely to be transmitted to offspring in mosquitoes that had a perturbed microbiome from antibiotic treatment compared to mosquitoes with their natural microbiota. Also, the fitness costs of Wolbachia infection disappeared in those reared under antibiotics, suggesting that other bacteria found in Anopheles’ microbiome were to blame for instable Wolbachia infections. Subsequent tests and genetic sequencing of the mosquito microbiome found that Asaia bacteria were the leading candidates for Wolbachia’s main microbial competitors. In the limited resource environment inside a mosquito, it is possible that the two types of bacteria compete resulting in poor uptake of Wolbachia infection when native Asaia are at high levels, but in the absence of competition post-antibiotic treatment, Wolbachia may be able to thrive. Future research should shed light on the mechanism behind the recent findings by Hughes and Rasgon.
In the mean time, the results of the study have important implications for addressing current barriers to using Wolbachia-infections to control wild populations of vectors and suggest one explanation for the patterns of prevalence seen in natural Wolbachia-infections. The findings have been published in the Proceedings of the National Academy of Sciences and can be found here: http://www.pnas.org/content/early/2014/08/06/1408888111.long
Synopsis written by Jo Ohm.
Written By: Hughes GL, Dodson BL, Johnson RM, Murdock CC, Tsujimoto H, Suzuki Y, Patt AA, Cui L, Nossa CW, Barry RM, Sakamoto JM, Hornett EA, & Rasgon JL
Paper Id: 10.1073/pnas.1408888111