Type I Fimbriae Promote the Virulence of <i>Serratia ureilytica</i> Strains that Cause Cucurbit Yellow Vine Disease While Impeding Biofilm Formation In Vitro

Cucurbit yellow vine disease (CYVD) is an emerging phloem disease in the United States that causes yield losses of up to 100% in squash, pumpkin, and watermelon. CYVD is caused by a group of closely related strains of <i>Serratia ureilytica</i>, a species comprising entomopathogenic, phytopathogenic, clinical, and environmental strains. One genomic trait distinguishing CYVD-causing <i>S. ureilytica</i> strains from non-CYVD <i>S. ureilytica</i> strains is the presence of a specific type I fimbrial locus, which we found to be uniquely plasmid-borne in CYVD strains. We investigated the contribution of this <i>fim</i> locus to CYVD by comparing the virulence of the wild-type CYVD strain Z07 to a deletion mutant Z07Δ<i>fim</i> that lacked the entire seven-gene <i>fim</i> locus. Infection assays with these strains showed that this locus contributes to virulence on squash (<i>Cucurbita pepo</i>) and does so by contributing to a higher probability of developing large populations in planta. Moreover, the Z07Δ<i>fim</i> mutant formed larger biofilms than the wild type in vitro, suggesting that these type I fimbriae interfere with biofilm formation. These results support a model in which biofilm development following <i>S. ureilytica</i> plant infection negatively impacts virulence, such as by restricting the number of sites accessed in the phloem tissue, thereby impeding spread and limiting colonization.