Background: Current Streptococcus pneumoniae vaccines selectively target capsular polysaccharide of specific serotypes, leading to an increase in nonencapsulated S. pneumoniae (NESp). Cocolonization by encapsulated pneumococci and NESp increases the opportunity for intraspecies genetic exchange. Acquisition of NESp genes by encapsulated pneumococci could alter virulence and help vaccine-targeted serotypes persist in the host.
Methods: Adhesion and invasion assays were performed using immortalized human pharyngeal or lung epithelial cells. In vivo models assessing murine nasopharyngeal colonization and pneumonia, as well as chinchilla otitis media (OM), were also used.
Results: Pneumococcal surface protein K (PspK) expression increased encapsulated pneumococcal adhesion and invasion of lung cells and enhanced virulence during pneumonia and OM. Additionally, PspK increased nasopharyngeal colonization, persistence in the lungs, and persistence in the middle ear when expressed in a capsule deletion mutant. Competition experiments demonstrated encapsulated pneumococci expressing PspK also had a selective advantage in both the lungs and nasopharynx.
Conclusions: PspK increases pneumococcal virulence during pneumonia and OM. PspK also partially compensates for loss of virulence in the absence of capsule. Additionally, PspK provides a selective advantage in a competitive environment. Therefore, acquisition of PspK increases encapsulated virulence in a condition-dependent manner. Together, these studies demonstrate risks associated with pneumococcal intraspecies genetic exchange.