A multilocus sequence typing (MLST) system for group B streptococcus (GBS) has been developed and validated on a global collection of human GBS strains isolated from carriage and invasive disease. A carriage study was performed, over a 3-year period, to establish the rate of carriage ofGBS in pregnant women in Oxford, UK. Invasive isolates were collected, prospectively and retrospectively over a similar time period. Twenty-one percent of women studied were asymptomatic carriers of GBS. The incidence of invasive GBS was 0.9/1000 live births in neonates and 6.11100,000 population >60 years. The population structure of GBS is best depicted. using MLST. as a network of related clusters indicating the presence of recomb inationa I events occurring in the population that interfere with a tree like branching structure of the population. A single hypervirulent clone ofGBS (ST-17 complex) is responsible for an excess of neonatal disease in Oxford (odds ratio 3.4). The possibility that a factor other than capsular type IIImay be responsible for virulence of this clonal complex in neonates is raised. Intriguingly this clonal complex was unique among human lineages in that it has emerged from bovine GBS. It was not however associated with increased invasiveness amongst adult (> 60 years). Further study ofthis hypervirulent clone of GBS is likely to contribute to the understanding of the pathogenesis of neonatal GBS disease.

Streptococcus pneumoniae (S.pneumoniae) is a respiratory pathogen associated with high morbidity and mortality worldwide. Nasopharyngeal colonisation is necessary for person to person transmission. Over 90 pneumococcal serotypes have been identified and it is estimated that at any one time, 60% of children carry the bacterium asymptomatically. Through the use of mathematical modelling, this research aims to investigate the competitive effects amongst serotypes and to quantify the likelihood of serotype replacement as a result of the introduction of the pneumococcal conjugate vaccine in Scotland.

Enterococci are intestinal facultative anaerobic strains which recognized as opportunistic pathogens. The ability to form biofilms is an important virulence trait that has been reported for Enterococci. Biofilm formation showed differences between E. faecalis strains. However, several factors were involved in this process e.g. the presence of virulence factors, hydrophobicity and heterogeneity. Interestingly, we demonstrated for the first time a biochemical test for a cell surface protein in biofilm formation: addition of the purified N domain of Esp (EspN) to E. faecium E1162Δesp resulted in the restoration of biofilm formation. Streptococcus bovis also, is an intestinal facultative anaerobic bacterium. This organism also has been reported as an opportunistic pathogen causing multiple diseases such as septicemia and endocarditis associated with colorectal cancer (CRC). Although the association of S. gallolyticus infection with CRC is a major issue, the mechanisms behind this link are still unclear. This link between CRC and the virulence of S. bovis strains was studied in more detail in a collaborative project with Dr Harold Tjalsma. The Tjalsma group mainly focussed on host-pathogen interactions, whereas we analysed biofilm formation of S. bovis strains as well as their pathogenicity using the in vivo C. elegans infection model. Our biofilm showed that S. bovis strains form biofilms particularly well on collagen-rich surfaces at least indicate why there is this association. C. elegans experiments also showed that pathogenicity of S. bovis strains is more similar to E. faecalis than to E. faecium in which both S. bovis and E. faecalis have a slow mode of killing that is absent in E. faecium. Full genome sequences of S. gallolyticus UCN34 strain have revealed the presence of a number of potential collagen-binding proteins (e.g., gallo_2179) that are related to the MSCRAMMs family. However, we successfully cloned the gallo_2179 gene in an enterococcal expression vector, and demonstrated transcribed in E. faecalis. Unfortunately, this strain did not form better biofilms on a collagen surface, suggesting either that not sufficient amount of the protein was made, or that the protein is not functional in E. faecalis. In addition, a bioinformatics analysis was performed to identify putatively secreted proteins in S. gallolyticus. Proteins that were expected to be found include for instance three collagen-binding proteins, amylase, tannase and beta lactamase.

This thesis explores aspects of Streptococcus pneumoniae (pneumococcus) epidemiology and control, in view of the possible widespread introduction of conjugate vaccines in England and Wales. A review and analysis of a range of different epidemiological data sources showed that the burden of pneumococcal disease in England and Wales is high and remains mostly a condition of the very young and the elderly. A meta-analysis demonstrated the effectiveness of the polysaccharide vaccine against invasive pneumococcal disease among healthy elderly, to whom vaccination was not recommended at the start of this work. Using this result, a costeffectiveness analysis assessed the economic acceptability of such a programme, from the public health perspective. A better understanding of pneumococcal carriage and transmission is required to assess the effectiveness and cost-effectiveness of mass vaccination strategies with the pneumococcal conjugate vaccine. A novel model framework was developed and fitted to a longitudinal dataset of carriage in UK families. The results demonstrated an inverse relationship between duration of carriage and age and highlighted the importance of both family size and composition for persistence in a household. Great dissimilarities were estimated among the specific serotypes in terms of transmissibility, duration of carriage and level of competition. Realistic age structured dynamic models were developed and used to investigate the impact of a range of vaccine strategies. The importance of serotype replacement, as a consequence of vaccination, was demonstrated. The economic acceptability of alternative interventions with the conjugate vaccine depended on the magnitude of its indirect effects. Herd immunity had a considerable impact on the overall cost-effectiveness of the programmes since it may substantially reduce the burden of disease in older age groups. However, serotype replacement may counterbalance this reduction and lead to a non cost-effective result.