Pseudomonas aeruginosa is an opportunistic pathogen and usually targets immunocompromised patients such as burn victims and patients with AIDS, cancer or cystic fibrosis (CF). Acquisition of this organism is associated with high mortality and can cause death within 24 hours. The main themes covered within this thesis are pathogenicity island characterisation in vitro and in vivo as well as investigation of in vivo virulence of P. aeruginosa. The rationale behind this focus is that 10-20% of the P. aeruginosa genome is variable between strains and large variable regions such as genomic (pathogenicity) islands are considered more likely to contribute to the differences in disease-causing ability between strains.\ud The first project covers the development of a novel generic yeast-based genomic island capture method, which enables a complete genomic island to be present within a cloning vector. It was used in the characterisation of genomic islands in both P. aeruginosa and Escherichia coli. A novel genomic island in E. coli was captured and characterised.\ud The second project investigates the contribution of two pathogenicity islands, PAPI- 1 and PAPI-2 to the in vivo virulence of P. aeruginosa PA14. Three pathogenicity island deletant isogenic mutants were tested for virulence in a murine acute respiratory model of infection developed for this project. The results showed that both pathogenicity islands contribute to virulence, but the presence of PAPI-2 is enough to maintain wild-type virulence.\ud The third project covers the exploration of the role of quorum-sensing in the virulence of P. aeruginosa LES; one of the transmissible epidemic strains and the most common strain recovered from CF patients across the UK. The project assessed whether over-expression of quorum-sensing products is a reliable indicator of increased virulence within a murine acute respiratory model of infection. The results showed in general, over-expressing mutants were more virulent than deficient mutants, but there was one exception, LESB58
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