The vector regulation hypothesis: dynamic competition between pathogen and vector behaviors constrains Xylella fastidiosa biofilm development in sharpshooter foreguts

Authors: Backus, Elaine; SHUGART, HOLLY J - Pennsylvania State University

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: One of the most destructive invasive plant pathogens in the world is Xylella fastidiosa (Xf), which causes devastating diseases such as citrus variegated chlorosis in South America, Pierce’s disease of grapevines in North America, and olive quick decline in Italy. Vector-transmitted, Xf is carried by sharpshooter leafhoppers and spittlebugs. One explanation to why Xf can easily invade new environments is that the bacterium can be acquired and inoculated by insects in a very short period. To better understand the pathogen transmission process, scanning electron microscopy was used to examine the foreguts of sharpshooters fed on Xf-infected grapevines for 4 and 7d. Results showed that multiple interrupted cycles of bacterial attachment and early stages of biofilm formation occurred in most areas of the foregut, while a few areas had rare but older, more developed colonies yet with clear indication of disturbance. This evidence indicates that accumulation of bacterial colonies was resisted by the insect, probably when biofilm formation reached a level that interfered with vector feeding, leading to behaviors such as the discharge of fluid from the vector's functional foregut and enzymatic salivation that likely loosen and eject Xf biofilm. Thus, vector acquisition and inoculation of Xf are dynamic, push-pull processes of interactions between bacteria and insect. This understanding of Xf biofilm formation aids ongoing development of resistance to Pierce's disease in grapevines and to diseases caused by Xf in other crops.

Technical Abstract: Xylella fastidiosa (Xf) bacteria form biofilm on the cuticular surfaces of the functional foregut (precibarium and cibarium) of its vectors, xylem fluid-ingesting sharpshooter leafhoppers and spittlebugs. While much is known about Xf biofilm development and maturation in vitro, little is known about these processes in vectors. RT (Real Time) -PCR was used to quantify Xf genomes daily in the functional foreguts of blue-green sharpshooters Graphocephala atropunctata, over 7 days of exposure to infected grapevines. Scanning electron microscopy (SEM) was used to examine Xf biofilm formation at 4 and 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 variability in quantity and location of bacterial attachment. Only early-stage biofilm formation occurred in low-turbulence areas of the cibarium, while high-turbulence areas of the cibarium and precibarium had rare but older, more developed macro-colonies. Biofilm was almost absent at 7 days but left behind adhesive material and remnants of prior colonization. Evidence supports the hypothesis that bacterial colonization was repeatedly interrupted and constrained by the vector. Behaviors such as egestion and enzymatic salivation likely can loosen and eject Xf biofilm, perhaps when profuse biofilm interfered with ingestion. Thus, vector acquisition of Xf is a dynamic and stochastic process of interactions between bacteria and insect. We further hypothesize for future testing that the insect can regulate this interaction. Deep understanding of Xf acquisition will aid the ongoing development of grapevine resistance to vector transmission of xylellae diseases.