Reactive oxygen species-mediated immunity against bacterial infection in the gut of cat fleas (Ctenocephalides felis)

Fleas (Order Siphonaptera) transmit numerous bacterial pathogens that cause severe human diseases (e.g., cat scratch disease, flea-borne spotted fever, murine typhus, plague). Because initial entry of these infectious agents occurs while blood feeding, the immune response in the flea gut is considered to be the first line of defense against invading microbes. However, relatively few studies have identified the flea immune molecules that effectively resist or limit infection in the gut. In other hematophagous insects, an immediate immune response to imbibed pathogens is the generation of reactive oxygen species (ROS). In this study, we utilized cat fleas (Ctenocephalides felis) to investigate whether oral infection with a well-known insect bacterial pathogen (Serratia marcescens) induces ROS synthesis in the flea gut, and whether production of ROS provides a defense mechanism against microbial colonization. Specifically, we treated fleas with an antioxidant to limit the number of free radicals in the digestive tract prior to infection, and then measured the following: S. marcescens infection loads, hydrogen peroxide (ROS) levels, and mRNA abundance of ROS signaling pathway genes. Overall, our data shows that ROS levels increase in response to infection in the flea gut, and that this increase helps to strengthen the flea immune response through the microbicidal activity of ROS.