The SmpA outer membrane lipoprotein of B. hyodysenteriae has several characteristics that indicate the potential to protect against swine dysentery (SD). It localises to the outer membrane and antibodies directed against SmpA can prevent the growth of B. hyodysenteriae in vitro. There is some variation observed in the distribution and expression of the SmpA lipoprotein, suggesting that vaccination with SmpA may not provide protection against challenge with a heterologous B. hyodysenteriae strain. This study has characterised the variation at the smpA locus, and in the process has identified a novel gene, smpB. There is very low similarity between smpB and smpA, with the exception of an identical lipoprotein signal sequence. This suggests that SmpB may be translocated to the outer membrane of B. hyodysenteriae in a similar fashion to SmpA. The results described in this thesis indicate that strains of B. hyodysenteriae harbour either smpA or smpB, but not both, explaining the earlier results of Turner et al. (1991). The presumed outer membrane location of SmpB lead to further investigations into its potential to protect mice from infection with B. hyodysenteriae. Swine Dysentery is a inflammatory disease of the swine colon. Therefore it is believed that a mucosal immune response may provide increased protection against challenge. In this study, vaccination of mice with recombinant SmpB elicited high levels of serum antibodies, induced the production of Interleukin-4 producing T lymphocytes and decreased the observed histological effects after challenge with virulent B. hyodysenteriae. In efforts to increase the protected conferred by vaccination with SmpB, recombinant Salmonella typhimurium STM-1 vaccines were created to express SmpB or deliver DNA vaccines encoding SmpB. Vaccination with these recombinant Salmonella vectors did not induce a measurable SmpB specific immune response. Macrophage survival and plasmid stability studies indicated that this was due to instability of the expression plasmids in STM-1. Although SmpB will only ever protect against strains of B. hyodysenteriae harbouring smpB, these results indicate that with further research, SmpB (and SmpA) may contribute to protection from SD. Toxin production is an important aspect of the pathogenesis of many pathogenic bacteria. Vaccination with attenuated toxins is commonly used to prevent disease. In this study, the B. hyodysenteriae â-haemolysin HlyA was used to vaccinate mice to determine the protection induced after challenge. Vaccination of mice with recombinant HlyA induced significant levels of serum antibodies and lowered the observed pathological effects after challenge of vaccinated mice with virulent B. hyodysenteriae. In an attempt to increase the mucosal immune response and therefore the protection afforded after vaccination with HlyA, recombinant S. typhimurium STM-1 strains were created to express HlyA or deliver DNA vaccines encoding HlyA. Similar to the recombinant STM-1 vaccines expressing SmpB, a HlyA specific immune response was not observed by ELISA or ELISPOT analysis. Plasmid stability trials revealed that the inability to induce a detectable HlyA specific immune response by recombinant STM-1 vaccination may be due to ins tability of the plasmids. Outer membrane proteins are often important components of vaccines against bacterial and viral pathogens. Considering the variation observed in the smpA locus in this study resulting in the identification of smpB, further investigation into the distribution and conservation of outer membrane encoding genes in B. hyodysenteriae strains was undertaken. In particular, the blpAEFG, vspABCD and vspEFGH clusters were analysed for their distribution. It was demonstrated that genes that are B. hyodysenteriae specific (vspABCD and vspEFGH) displayed higher levels of polymorphism than those that are distributed amongst non-pathogenic species, such as B. innocens (which contains blpAEFG). This suggests that the variation in the vspABCD and vspEFGH clusters amongst B. hyodysenteriae strains may be a result of the exposure to the host immune system. Further investigation was undertaken by PFGE analysis and 2D-gel electrophoresis, to analyse genomic and proteomic variation at a global level. Although strains of B. hyodyse nteriae produced several different electrophoretic types (ET) upon PFGE analysis, only limited correlation between the PFGE ET, the polymorphisms in vspABCD and vspEFGH and the presence of smpA/smpB were observed. 2D-gel electrophoresis analysis of outer membrane preparations of two B. hyodysenteriae strain revealed several distinct differences in the outer membrane between B. hyodysenteriae strains. The observed differences in the proteins contained in the outer membrane of B. hyodysenteriae is important for vaccine design, as the induction of cross protection between strains of B. hyodysenteriae is essential for a effective vaccine.