Determination of the role of the microbiota in the fitness of Amblyomma aureolatum ticks and in the acquisition and transmission of Rickettsia rickettsii

Abstract

Rocky Mountain spotted fever is an acute infectious disease present throughout the American continent. In Brazil, it is known as Brazilian spotted fever (FMB - Febre Maculosa Brasileira), and, due to its nonspecific symptomatology, it has high lethality rates, reaching 70% in the state of São Paulo. FMB is caused by the bacterium Rickettsia rickettsii and its transmission to humans occurs through the bite of infected ticks. In Brazil, R. rickettsii is transmitted by two tick species: Amblyomma sculptum and Amblyomma aureolatum. Although both species are important vectors of R. rickettsii, A. sculptum has a lower susceptibility to infection than A. aureolatum. Previous studies from our research group have shown that these susceptibility differences correlate with a distinct transcriptional response from the tick gut in response to R. rickettsii infection. While most of the genes modulated by the infection were upregulated in the gut of A. sculptum, including components of the immune system, in the gut of A. aureolatum the modulated genes were mostly repressed. We also observed that the total number of bacteria of the gut microbiota differs between the two species: while A. sculptum has a reduced microbiota, A. aureolatum has a prominent microbiota, which is mostly composed of bacteria in the Francisella genus. These results suggest that Francisella desensitizes the immune system of A. aureolatum gut, forming a favorable environment for R. rickettsii colonization. Based on the results previously obtained, the current project has as main aim to determine the relationship between the microbiota and the fitness and vector competence of A. aureolatum to R. rickettsii. To this end, adult females of A. aureolatum will be treated with antibiotics to obtain a sterile strain or a strain with reduced microbiota. Larvae, nymphs and adults will have their fitness evaluated at each stage. To assess vector competence, the larvae obtained will be fed on guinea pigs infected or not with R. rickettsii. Larvae, nymphs and adults will have their total bacterial load and R. rickettsii load determined. Additionally, the animals on which ticks will feed will be analyzed for clinical signs of FMB. As a control, the procedures will be performed with ticks not treated with antibiotics. Another stage of the project aims to identify cultivable components of the A. aureolatum microbiota, which will be carried out with the support of a BEPE fellowship (to be required in the future). It is expected that at the end of this project, it will be possible to understand the importance of the microbiota for the fitness and vector competence of A. aureolatum to R. rickettsii. (AU)