D3516: The vector regulation hypothesis: Dynamic competition between pathogen and vector behaviors regulates Xylella fastidiosa biofilm development in sharpshooter foreguts

Xylella fastidiosa (Xf) bacteria form biofilm on cuticular surfaces of the functional foregut (precibarium and cibarium) of its vectors, xylem fluid-ingesting sharpshooters and spittlebugs. While much is known about the time course of Xf biofilm development and maturation in vitro, little is known about these processes in vectors. RT-PCR was used to quantify Xf genomes daily in the functional foreguts of blue-green sharpshooters, Graphocephala atropunctata (Say) (Hemiptera:  Cicadellidae), over 7 days of exposure to Xf- infected grapevines. Scanning electron microscopy (SEM) was used to examine Xf biofilm formation at the middle (4 days) and end (7 days) of that time course.  PCR showed populations building and reducing over a 4-day cycle.  SEM revealed that foreguts at 4 days showed signs of iterative, interrupted cycles of bacterial attachment and only early-stage biofilm formation in low-turbulence areas of the cibarium, while high-turbulence areas of the cibarium and precibarium had rare but older, more developed macro-colonies yet with clear evidence of disturbance. Biofilm was almost absent at 7 days but left behind adhesive material and other remnants of prior colonization. Evidence supports the hypothesis that bacterial colonization was repeatedly interrupted by the vector.  Behaviors such as enzymatic salivation and egestion likely can loosen and eject Xf biofilm, presumably when profuse biofilm interfered with ingestion. Thus, vector acquisition of Xf is a dynamic, competitive process of interactions between bacteria and insect. Deep understanding of Xf acquisition will aid the ongoing development of grapevine resistance to vector transmission of xylellae diseases.