Updated project metadata. Transmission of Yersinia pestis by fleas depends on the formation of condensed bacterial aggregates embedded within a gel-like matrix that localize to the proventricular valve in the flea foregut and interfere with normal blood feeding.The matrix of the bacterial aggregates is complex,containing large amounts of protein and lipid (particularly cholesterol) derived from the flea's blood meal. The goal of this study was to biochemically characterize the EPS of the Y. pestis aggregates that form after infection and to identify factors required for this initial stage of biofilm formation.