AbstractYersinia pestis, the causative agent of plague, encodes for a large repertoire of pili, which are hair-like fibers that extend out from the bacterial surface. Pili formed by the chaperone/usher pathway are adhesive structures that contribute to virulence in many Gram-negative bacteria. The Y. pestis genome contains eleven chaperone/usher operons. I sought to understand what roles these pathways have on the ability of Y. pestis to bind to host cells, cause disease, and contribute to virulence. I initially examined a panel of Y. pestis mutants containing deletions of the usher genes for six of these chaperone/usher pathways. I found that at least three of the pathways aided in adhesion to mammalian cells. When ushers y0350 or y1858 were deleted, there was a decrease in adhesion to all host cell types examined compared to the wild-type Y. pestis strain. When usher y1871 was deleted, there was a decrease in adhesion to macrophages. In addition, when mice were infected intranasally with the KIM5+ deletion strains, there was a significant attenuation in virulence of the mutants lacking the three previously mentioned ushers. To completely investigate the chaperone/usher pathways of Y. pestis, I constructed a new panel of mutants containing full deletions of each of the nine uncharacterized chaperone/usher pathways. Adhesion studies were repeated and in agreement with the previous work, when pathways y0348-y0352 or y1858-y1862 were knocked out, there was a significant decrease in binding to all cell types, and when pathway y1869-y1873 was knocked out, there was a significant decrease in binding to macrophages. In addition, a new pathway, y4060-y4063, was identified as important for binding a number of cell types. Finally, I examined whether these pathways facilitate the delivery of virulence factors, termed Yops, to host cells. These studies revealed that Y. pestis mutants containing deletions of pathways that are important for adhesion to host cells are also impaired in Yop delivery. Together, these studies showed that at least four of the chaperone/usher pathways of Y. pestis aid in binding and delivery of Yops to a variety of host cells, and they contribute to the virulence of Y. pestis via the pneumonic route.