Identification of niche-specific virulence factors via experimental evolution of Streptococcus pneumoniae

Streptococcus pneumoniae (the pneumococcus) is an important human pathogen, adept at colonising various ecological niches within the host. Colonisation of the nasopharynx, followed by asymptomatic carriage and non-inflammatory clearance is the predominant outcome of infection, but diverse disease manifestations including pneumonia, septicaemia and meningitis occur in a minority of individuals. Through experimental evolution of pneumococci in mouse disease models, we are investigating the genetic basis of the niche adaptations that enable pneumococci to switch from a commensal lifestyle in the nasopharynx, to a pathogenic phenotype in the lungs, brain or blood. Experimental evolution was carried out via serial passage of pneumococci separately through pneumonia and nasopharyngeal carriage mouse models, to generate lineages adapted to the lung and nasopharyngeal environments, respectively. Starting from a non-passaged (lab adapted) isolate, ten independently-evolved lineages of lung-adapted pneumococci have been generated, each having been passaged 20-times through a mouse pneumonia model. Pneumococci recovered from the infected lungs were used to inoculate further mice for the next passage round. We will present the results from whole genome sequencing and phenotypic analysis of lung-passaged bacterial isolates including growth characteristics, toxin production, adherence and invasion with lung epithelial cells. We will also describe how the acquired pneumococcal adaptations, which facilitate survival in the lung environment, can influence bacterial gene expression during exponential growth. These studies will provide insight into genetic changes associated with pneumococcal commensal to pathogen switch. Identifying such genetic determinants of virulence will be valuable for the development of vaccine candidates and targets for therapeutic intervention.