Gene expression patterns in flea vectors of Yersinia pestis

Plague (Yersinia pestis) is transmitted by >60 flea species, yet little is known about the determinants of vector competence. We hypothesize that fleas alter their gene expression in response to Y. pestis infection, and that differentially expressed genes contribute to vector competence and affect transmission. We utilized suppression subtractive hybridization to identify genes specifically active in infected Xenopsylla cheopis (rat flea, an efficient vector) and Ctenocephalides felis (cat flea, a poor vector). Among these, we isolated a heat-shock protein that could be part of a stress response. Oral and hemocoel infection in X. cheopis increased the transcription of defensin, an antibacterial peptide previously identified in salivary glands. Although fleas produced higher levels of acid-extractable antibacterial peptides in response to an uninfected blood meal, defensin transcription was not induced by blood feeding alone. Defensin may help contain Y. pestis to the digestive tract and limit the growth of other bacteria.