199 - Streptococcus pneumoniae

Abstract

Streptococcus pneumoniae is a microaerophilic gram-positive diplococcus that is a leading cause of pneumonia and related death in young children and older adults worldwide, as well as of bacteremia, meningitis, and otitis media. The over 90 serotypes of the surface polysaccharide capsule serve the organism’s major virulence factors, which, in concert with several intercalating surface proteins, support evasion of phagocytosis. Other proteins facilitate adherence to the respiratory epithelium and perturb complement activation. This pathogen is confined to humans and spreads from one to another by intimate contact. Colonization of the nasopharynx generally precedes infection and can elicit local and systemic antibodies. Pneumococcal disease can begin by contiguous spread from the nasopharynx to the middle ear, sinuses, and intracranial cavity or by microaspiration into the lungs to cause pneumonia. Invasion into the bloodstream can spread to the choroid plexus, joint space, or the peritoneal cavity. Persons with low levels of antibody and complement, HIV infection, and underlying organ dysfunction, particularly chronic lung disease, are at increased risk of pneumococcal disease. Current vaccines are comprised of multiple capsular polysaccharides, alone conjugated to a protein, which elicit capsule-specific antibodies that most consistently prevent bacteremic disease. Conjugate vaccines show less-prominent but significant protection against pneumonia in children and adults. The effective reduction in the burden of pneumococcal disease by vaccine is tempered in part by the emergence of nonvaccine replacement serotypes. Ongoing vaccine development is directed to increasing the number of serotypes in newer vaccines as well as the inclusion of conserved proteins, such as pneumolysin, alone or as polysaccharide conjugates.