Pneumococcal pneumonia and invasive pneumococcal disease: immunopathogenesis and diagnosis

Streptococcus pneumoniae is the most common aetiology of community-acquired pneumonia (CAP). It has many virulence factors, the most important being a polysaccharide capsule (Cps). There are 97 different serotypes of pneumococcal based on Cps which include both colonization and invasive serotypes. Pneumococcal pneumonia may exist as a result of either aspiration of bacteria in the nasopharynx or inhalation of droplet nuclei which contains bacteria until they reach the lower respiratory tract. This condition will activate both innate and adaptive immune system. The diagnosis of pneumococcal pneumonia is established in a patient who has the signs and symptoms of pneumonia, accompanied by the detection of S. pneumoniae in microbiology examination. Pneumococcus may also penetrate into a normally sterile site such as bloodstream, meninges, and pleural cavity, and infection of pneumococcus in those sites are defined as an invasive pneumococcal disease (IPD). High bacterial load, dysfunction of the immune system, and co-colonization of another microorganism may also lead to IPD.