The type III secretion system and its role in the multicellular behaviour of Yersinia species

Pathogenic Yersinia translocate Yersinia outer protein (YOP) effectors directly into eukaryotic cells via a virulence plasmid (pYV/pCD1)-encoded type III secretion system (T3SS) injectisome, where they interfere with host signalling pathways and inhibit inflammatory responses. A temperature of 37°C is required for injectisome assembly and YOP export is triggered by host cell contact or chelation of calcium ions. Yersinia pseudotuberculosis and Yersinia pestis both utilise N-acylhomoserine lactone (AHL) -dependent quorum sensing (QS), a process of signal molecule mediated cell-to-cell communication for coordinating gene expression within a bacterial population. In Y. pseudotuberculosis, T3S, motility, auto-aggregation (AAg) and biofilm formation on Caenorhabditis elegans are all under QS control. At 37°C, Y. pseudotuberculosis exhibits a T3SS-dependent AAg phenotype that is subject to AHL-mediated negative regulation. The rate of AAg and the size of the auto-aggregates was also found to be dependent on QS. The aims of this thesis were to determine the specific proteins responsible for this phenotype by screening T3SS mutants and assessing their ability to auto-aggregate. The adhesin YadA is a known AAg factor, however T3SS-dependent AAg was found to be YadA-independent. Previously mutation of both Y. pseudotuberculosis AHL synthase genes, ytbI and ypsI, was shown to attenuate biofilm formation on the living (biotic) surface of C. elegans in a pYV-dependent manner. Since this finding suggested a role for the T3SS in biofilm formation, mutations in yopB, yopH, yscF, yscJ and lcrV were introduced into the AHL-negative synthase mutant and they were screened for their ability to form biofilm