Xylella fastidiosa bacteria (a) are typically observed embedded in a blanket or involved by a substance (b) of unknown nature inside xylem vessels of citrus or coffee . It is not known whether the substance is produced by the bacteria as a natural process in the colonization of the vessel, or it is produced by the vessel as a defense against colonization, in an attempt to isolate the invaders inside the affected vessel (c). The appearance is typically of a cake roll  or pie where the bacteria  are the filling.
Xylem vessels possess pores or pits (d). The pits are frequently larger than bacteria in dimension. However, bacteria are not allowed through xylem pits because of an existing membrane between pits of adjacent xylem vessels with a structure similar to cell membranes which, when intact, are only permeable to small molecules. There is no evidence yet of sufficient production and/or secretion of specific cell membrane digestion enzymes by X. fastidiosa. Therefore, lateral translocation of the bacteria between adjacent vessels seems rare. Typically, vessels completely taken by bacteria appear adjacent to perfectly clean vessels under electronic microscope observation. Longitudinal sections above, 2,100X, and below, 5,400X.

5400X