Streptococcus pneumoniae ('the pneumococcus') is a frequent asymptomatic coloniser of the nasopharynx, from where it may disseminate to cause life-threatening infections including pneumonia, bacteraemia, and meningitis. Pneumococcal disease remains a leading cause of global mortality despite the use of safe and effective pneumococcal conjugate vaccines (PCVs). Bacteriocins are antimicrobial peptides that are produced by bacteria to target competitor bacteria within the ecological niche. Twenty pneumococcal bacteriocins have been characterised in silico, but their role in competition within the nasopharynx is not yet understood.

In the first part of this project, I studied the distribution of bacteriocin genes in two large genomic datasets (>5,000 pneumococcal genomes in total) sampled from Iceland and Kenya. The distribution of some bacteriocins differed by location, between pneumococci recovered from carriage and disease, and between pneumococci recovered before and after the introduction of PCVs. These observations were largely explained by the association of bacteriocins with clonal complexes and suggested that there were different competition dynamics among pneumococci.

A functional model of the streptococcin bacteriocins was generated using structural predictions. This informed further genomic studies, which observed genetic heterogeneity in the streptococcins. A dataset of >1,800 genomes from non-pneumococcal streptococci was screened for streptococcins, which were commonly harboured by viridans streptococci. There was evidence that the streptococcin diversification was driven by horizontal exchange between pneumococci and non-pneumococcal streptococci.

In the final part of the project, the streptococcins were studied experimentally. A streptococcin toxin was isolated for the first time using a recombinant expression and purification method. The streptococcin was used in susceptibility assays against a panel of pneumococci and non-pneumococcal streptococci. Preliminary results suggested that the streptococcin had activity against some of the test strains.

Results presented in this thesis expand our understanding of pneumococcal bacteriocins and will be used to inform further genomic and experimental studies.