College of Graduate Studies: Theses & Dissertations

Term of Award

Spring 2026

Degree Name

Master of Science in Biology (M.S.)

Document Type and Release Option

Thesis (open access)

Copyright Statement / License for Reuse

Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.

Department

Department of Biology

Committee Chair

Lisa Brown

Committee Member 1

Josh Gibson

Committee Member 2

Stephen Greiman

Abstract

Ctenocephalides felis are vectors of several bacterial pathogens that cause disease in humans and animals, including Bartonella henselae and Rickettsia felis. Successful pathogen transmission requires that microbes survive the immune defenses of the flea digestive tract following blood feeding. In insects, humoral immune responses are regulated primarily by the Toll and immune deficiency (IMD) signaling pathways, with the IMD pathway serving as the principal defense against Gram-negative bacteria. Although genomic and transcriptional evidence indicate that cat fleas encode and express components of the IMD signaling pathway, the functional roles of individual pathway components within the flea digestive tract remain poorly understood. The objective of this study was to determine how chemical inhibition of a core component of the IMD pathway affects antibacterial activity in the flea digestive tract. IMD signaling was suppressed using the NF-κB signaling inhibitor IKK16, required for activation of the transcription factor Relish. To confirm that the inhibitor did not directly affect bacterial growth, in vitro assays were conducted using the Gram-negative bacterium Serratia marcescens and the Gram-positive bacterium Micrococcus luteus. The effects of IMD inhibition on flea immune responses were then evaluated by measuring antibacterial activity of flea gut extracts in vitro, quantifying bacterial infection intensity in vivo following infected blood meals, and assessing transcript levels of the antimicrobial peptide defensin using quantitative PCR.

IKK16 did not directly affect bacterial growth in vitro. However, gut extracts from fleas exposed to IKK16 supported significantly greater bacterial growth than extracts from control groups, indicating reduced antibacterial activity following inhibition of IMD signaling. Despite this reduction in antibacterial activity, bacterial infection intensity in vivo did not differ among treatment groups. In addition, IKK16 significantly reduced defensin transcript levels during blood feeding but did not alter defensin expression following bacterial challenge. Together, these findings demonstrate that IMD signaling contributes to the regulation of antimicrobial defenses in the flea digestive tract but may not be the primary determinant of early bacterial colonization in vivo. These results highlight the complexity of immune regulation in insect vectors and emphasize the importance of evaluating molecular immune responses in the broader context of infection dynamics and vector competence.

Research Data and Supplementary Material

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