Transcriptomic Profiling of the Digestive Tract of the Rat Flea, Xenopsylla cheopis

Yersinia pestis, the causative agent of plague, is a highly lethal pathogen transmitted by the bite of infected fleas. Once ingested by a flea, planktonic Y. pestis establish a replicative niche in the gut and produce a biofilm that promotes foregut colonization and transmission. Xenopsylla cheopis, the rat flea, is an important vector to several zoonotic bacterial pathogens including Y. pestis. Some fleas naturally clear themselves of infection, however, the physiological and immunological mechanisms by which this occurs are largely uncharacterized. To address this, RNA was extracted, sequenced, and distinct transcript profiles were assembled de novo from X. cheopis digestive tracts isolated from fleas that were either: 1) not fed for 5 days 2) fed sterile blood or 3) fed blood containing ~5x108 CFU/ml Y. pestis KIM6+. Analysis and comparison of the transcript profiles resulted in identification of 23 annotated (and 11 unknown or uncharacterized) digestive tract transcripts that comprise the early transcriptional response of the rat flea gut to infection with Y. pestis. The data indicate that production of antimicrobial peptides regulated by the immune-deficiency pathway (IMD) is the primary flea immune response to infection with Y. pestis. The remaining infection-responsive transcripts, not obviously associated with the immune response, were involved in at least one of 3 physiological themes: 1) alterations to chemosensation and gut peristalsis; 2) modification of digestion and metabolism; and 3) production of chitin-binding proteins (peritrophins). Despite producing a number of peritrophin transcripts shortly after feeding, including a subset that were infection-responsive, no peritrophic membrane was detectable by histochemistry or electron microscopy of rat flea guts for the first 24 hours following blood-feeding or infection.