A significant human pathogenic bacterium, S. pneumoniae was recognized as a major cause of pneumonia in the late 19th century, and is the subject of many humoral immunity studies.
Streptococcus pneumoniae or pneumococcus is a Gram-positive, alpha-hemolytic, aerotolerant, anaerobic member of the genus Streptococcus.
S. pneumoniae resides asymptomatically in the nasopharynx of healthy carriers. However, in susceptible individuals, such as elderly and immunocompromised people and children, the pathogen can spread to other locations and cause disease. S. pneumoniae is the main cause of community acquired pneumonia and meningitis in children and the elderly, and of septicemia in HIV-infected persons.
Despite the name, the organism causes many types of pneumococcal infections other than pneumonia. These invasive pneumococcal diseases include acute sinusitis, otitis media, conjunctivitis, meningitis, bacteremia, sepsis, osteomyelitis, septic arthritis, endocarditis, peritonitis, pericarditis, cellulitis, and brain abscess.
S. pneumoniae is one of the most common causes of bacterial meningitis in adults and young adults, along with Neisseria meningitidis, and is the leading cause of bacterial meningitis in adults in the USA. It is also one of the top two isolates found in ear infection, otitis media. Pneumococcal pneumonia is more common in the very young and the very old.
S. pneumoniae can be differentiated from Streptococcus viridans, some of which are also alpha-hemolytic, using an optochin test, as S. pneumoniae is optochin–sensitive. S. pneumoniae can also be distinguished based on its sensitivity to lysis by bile, the so-called “bile solubility test”. The encapsulated Gram-positive coccoid bacteria have a distinctive morphology on Gram stain: lancet-shaped diplococci. They have a polysaccharide capsule that acts as a virulence factor for the organism; more than 90 different serotypes are known, and these types differ in virulence, prevalence, and extent of drug resistance.
The polysaccharide vaccine most commonly used today. consists of purified polysaccharides from 23 serotypes (1, 2, 3, 4, 5, 6b, 7F, 8,9N, 9V, 10A, 11A, 12F, 14, 15B, 17F, 18C, 19F, 19A, 20, 22F, 23F and 33F). Immunity is induced primarily through stimulation of B-cells which release IgM without the assistance of T cells.
This immune response is less robust than the response provoked by conjugated vaccines, which has several consequences. The vaccine is ineffective in children less than two years old, presumably due to their less mature immune systems. Non-responders are also common amongst older adults. Immunization is not lifelong, so individuals must be re-vaccinated every 5–6 years. Since no mucosal immunity is provoked, the vaccine does not affect carrier rates, promote herd immunity, or protect from upper or lower respiratory tract infections. Finally, provoking immune responses using unconjugated polysaccharides from the capsules of other bacteria, such as H. influenzae, have proven significantly more difficult.