Interaction of isogenic Streptococcus pneumoniae capsule mutants with components of the human (mucosal) immune system

Abstract

Streptococcus pneumoniae is a major contributor to death of children worldwide. The bacteria can cause pneumonia, otitis media and meningitis, causing severe illness or death. It is estimated that between 8-12% of all deaths of children between the ages of 1 and 59 months is caused by S. pneumococcus infection. In order to efficiently infect its hosts, S. pneumococcus makes use of a capsule, a sugar-based coating surrounding the bacteria. The capsule provides the bacteria with protection against various components of the immune system by blocking the function of antibacterial factors (complement) and the clearance of the bacteria by immune cells. Because of this importance for the bacterium’s survival, the capsule is also used to distinguish different strains of S. pneumococcus in a clinical setting. In this study, we make use of capsule mutants; a single strain of S. pneumococcus carrying capsule types derived from strains found in the human population. By using these capsule mutants, we were able to find differences between capsules with regards to the binding to human airway cells, amount of complement deposited on the bacteria and the resistance against clearance by the human immune system. While the amount of binding and deposition of complement did not show a relation with the frequency of causing disease reported for the capsule serotypes, the resistance against clearance by the immune system did seem to have a relation with how often a strain is able to infect children. We hope that the results of this research can be used to obtain a better understanding of the function of the S. pneumococcus capsule.