Investigation of virulence gene regulation in Streptococcus pneumoniae

Abstract

Streptococcus pneumoniae is an important human pathogen in all age groups worldwide that causes a variety of diseases, ranging from life threatening septicaemia and meningitis to less severe sinusitis and otitis media. The factors that determine the virulence of S. pneumoniae are very complex but a key aspect of the organism's disease causing potential is the ability of the bacteria to regulate virulence factor expression and activity. In this study two main approaches were taken to investigate virulence gene expression in S. pneumoniae. Firstly, the feasibility of Recombinase based In vivo Expression Technology, RIVET, for use in S. pneumoniae to study gene expression in vitro, and then in vivo was assessed. However, the system was found to be unsuitable for use in this study. Secondly, the requirement for and the role of virulence gene regulators identified by Signature Tagged Mutagenesis were investigated. The requirement for different virulence gene regulators varied according to the murine model of infection used. Two of the regulators, MgrA and RlrA, were essential for nasopharyngeal carriage and production of pneumonia in mice by serotype 4 S. Pneumoniae. Both were shown to control the transcription of genes of a newly described pathogenicity islet, PPI2, encoding RlrA and proteins predicted to act at the bacterial cell surface. The PPI2 genes rlrA and rrgA were shown to be required for adhesion of serotype 4 S. pneumoniae to human epithelial cells and PPI2 gene expression was affected by the gaseous composition of the growth environment in an MgrA dependent manner. The distribution of MgrA, RlrA and PPI2 varied between clinical S. pneumoniae isolates emphasizing the likelihood of a different repertoire of virulence genes and regulators amongst different serotypes and strains of this important human pathogen.