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.

跳蚤（虹吸目）传播多种细菌病原体，导致严重的人类疾病（例如猫抓病、跳蚤斑疹热、鼠斑疹伤寒、鼠疫）。由于这些传染性病原体的初始进入发生在吸血时，跳蚤肠道中的免疫反应被认为是抵御入侵微生物的第一道防线。然而，相对较少的研究已经确定了有效抵抗或限制肠道感染的跳蚤免疫分子。在其他食血昆虫中，对吸收病原体的直接免疫反应是产生活性氧 (ROS)。在这项研究中，我们利用猫蚤（Ctenocephalides felis）来研究口腔是否感染了一种众所周知的昆虫细菌病原体（Serratia marcescens ) 诱导跳蚤肠道中的 ROS 合成，以及 ROS 的产生是否提供了一种抵御微生物定植的防御机制。具体来说，我们在感染前用抗氧化剂处理跳蚤以限制消化道中自由基的数量，然后测量以下内容：粘质链球菌感染负荷、过氧化氢 (ROS) 水平和 ROS信号通路基因的mRNA 丰度. 总体而言，我们的数据显示 ROS 水平随着跳蚤肠道中的感染而增加，并且这种增加有助于通过 ROS 的杀微生物活性加强跳蚤免疫反应。