Characterizing Anopheles Gambiae Susceptibility to Oral Infections with a Panel of Human Bacterial Pathogens

Abstract

The mosquito midgut is a key organ associated with multiple physiological functions including development, digestion, immunity and reproduction. In mosquito vectors of diseases, all transmitted microbes including viruses and parasites start their journey in the midgut where they interact with several chemical and physical barriers that function to restrain microbial invasion of the host. In the malaria vector Anopheles gambiae, the physiological responses of the midgut epithelium as well as local barriers, such as the peritrophic matrix and barrier epithelial, are known to impact the development of early stages of Plasmodium parasites. However, the midgut responses to bacterial pathogens are not very well understood as those to malaria parasites, and have mainly focused on the use of the Gram-negative bacterium S. marcescens (Sm) as an oral pathogen to establish midgut infections. In this project, we aim to study mosquito susceptibility to oral infections with human bacterial pathogens known to cause gut infections, in order to determine whether these are virulent to the insect and, hence, can be used as tools to deepen our understanding of the mosquito gut physiologic responses to infection. The results show that several human pathogens including Pseudomonas aeruginosa, Salmonella Typhi, Klebsiella pneumoniae and Escherichia coli can indeed cause mosquito mortality when acquired through the oral route, however none of the tested human pathogens persisted in the mosquito midgut beyond 4 days from feeding suggesting that the midgut microenvironment is not permissive for colonization by these bacteria. We also show that oral infections with a particularly virulent P. aeruginosa sequence type 309 and the insect gut pathogen Erwinia carotovora carotovora significantly increased the number of cells undergoing enhanced endoreplication in the proventriculus but not in the midgut (anterior and posterior), further supporting the recognized immune defensive role of the proventriculus. None of the oral infections triggered significant cell division in midgut regenerative cells in PH3 staining, supporting previous studies in A. gambiae showing that chemical and microbial damage of the midgut does not trigger significant proliferation of regenerative cells. In summary, our data reveal that human bacterial pathogens can cause mosquito mortality when acquired orally despite their transient colonization of the midgut indicating that they might be causing irreversible damage. A genome wide transcriptomic approach is underway to identify the functional groups of genes whose expression may be altered by these oral pathogens, in order to gain better insight into mosquito midgut physiological responses to infection.