The choline-binding proteins PspA, PspC and LytA of Streptococcus pneumoniae and their role on host cellular adhesion and damage
Streptococcus pneumoniae is a Gram-positive opportunistic pathogen that can colonize the upper respiratory tract. It is a leading cause of a wide range of infectious diseases, including community-acquired pneumonia, meningitis, otitis media and bacteraemia. Pneumococcal infections cause 1-2 million deaths per year, most of which occur in developing countries, where this bacterial species is probably the most important pathogen during early infancy. Here, we focused on choline-binding proteins (CBPs), i.e., PspC, PspA and LytA, and their integration into and interaction with the cell wall of S. pneumoniae. The three pneumococcal proteins have different surface-exposed regions but share related choline-binding anchors. These surface-exposed pneumococcal proteins are in direct contact with host cells and have diverse functions. PspC and PspA bind several host plasma proteins, whereas LytA plays a role in cell division and the lytic phase. We explored the role of the three CBPs on adhesion and pathogenicity in a human host by performing relevant imaging and functional analyses, such as electron microscopy, confocal laser scanning microscopy and functional quantitative assays targeting biofilm formation and the haemolytic capacity of S. pneumoniae. In vitro biofilm formation assays and electron microscopy experiments were used to examine the ability of knockout mutant strains lacking the lytA, pspC or pspA genes to adhere to surfaces. The mutant strains were compared with the S. pneumoniae D39 reference strain. We found that LytA plays an important role in robust synthesis of the biofilm matrix. PspA and PspC appeared crucial for the haemolytic effects of S. pneumoniae on human red blood cells. Furthermore, all knockout mutants caused less damage to endothelial cells than wild-type bacteria, highlighting the significance of CPBs for the overall pathogenicity of S. pneumoniae. Hence, in addition to their structural function within the cell wall of S. pneumoniae, each of these three surface-exposed CBPs controls or mediates multiple steps during bacterial pathogenesis.