Adhesion and biofilm formation of Streptococcus pneumoniae

Abstract

The pneumococcus is a commensal bacterium of the human respiratory tract, but it can invade the sterile sites of the host, causing sometimes lethal disease. The aim of this study was to investigate factors that contribute to the cellular adhesion and invasion potential of S. pneumoniae at the early stages of infection, in both nasopharyngeal carriage and invasive disease. Phenotypic profiling of representative isolates of 15 pneumococcal serotypes of clinical relevance was undertaken, to understand the range of adhesion potentials of the pneumococcus to A549 airway epithelial cells. In parallel, phenotypic profiling of 20 closely related pneumococcal lineages from a previous experimental evolution study was performed. All pneumococcal strains were able to adhere to A549 cells but intracellular invasion was a rare event. The adhesion and invasion potential of strains was affected by capsule type and capsular thickness and by the production of the cholesterol-dependent cytolytic toxin pneumolysin. The ability to form biofilms is key to pneumococcal colonisation, which is itself a key step in the progression towards pneumococcal diseases. A positive correlation was observed between the ability of clinical strains to form biofilms and their adhesion potential. Pneumococcal proteins contributing to biofilm lifestyles were identified using M/S comparison of pneumococci during biofilm and planktonic growth. This was undertaken with strains identified as strong biofilm formers, and included serotypes 7F and 19A and two serotype 2 D39-derived strains from a previous experimental evolution study. The abundance of a set of functionally related proteins was increased during biofilm growth, relative to in planktonic cultures. Data showed that pneumococci in biofilms are likely virulence-attenuated compared to planktonic pneumococci, due to decreased production of virulence proteins such as HtrA and pneumolysin (Ply). The upregulated proteins in biofilm samples were those associated with adhesion such as LytC and PsaA, whilst those associated with virulence and toxicity were less abundant in biofilms samples. Collectively, this study highlights the diverse phenotypes that contribute to the adhesion potential of pneumococci. Furthermore, it identifies proteins that are markedly upregulated in biofilms. Such factors might be attractive targets for future therapeutic interventions.