• Streptococcus pneumoniae, or the pneumococcus, is a Gram-positive oval or lancet-shaped coccus typically arranged as a diplococcus or in short chains.
• Virulent strains are encapsulated. There are currently 90 serotypes of capsules used for serologic classification.
• Facultative anaerobe (def).

Habitat

• Colonizes the nasopharynx in humans.

Source

• Pneumococci are frequently found as normal flora of the nasopharynx of healthy carriers. From 10% to 40% of adults carry the bacterium in the nasopharynx.

Epidemiology

• In the U.S., they are the most common cause of community-acquired pneumonia requiring hospitalization, causing around 500,000 cases per year and usually occuring as a secondary infection in the debilitated or immunocompromised host.
• In the U.S., the pneumococci cause over 7,000,000 cases of otitis media (def) per year, are the leading cause of sinusitis in people of all ages, and are responsible for 55,000 cases of bacteremia (def).
• Pneumococci are also responsible for approximately 6000 cases of meningitis (def) a year in the U.S., being the most common cause of meningitis in adults and children over 4 years of age.

Clinical Disease

• An opportunistic bacterium causing otitis media (def) and sinusitis, typically following a viral infection of the upper respiratory tract.
• Pneumonia (def) occurs when the bacteria grow in the alveoli (def) of the lungs. Usually abrupt onset with sustained fever of 39°C to 41°C accompanied by a severe shaking chill. Typically patients have a productive cough with blood-tinged sputum and pleurisy (def). The overall mortality rate is approximately 5% but is influenced by the strain of the pneumococcus, the age of the patient and other underlying diseases..
• Meningitis (def) occurs when the pneumococcus spreads into the central nervous system from infected sinuses, ears, or blood or following head trauma. Occurs at all ages but is mostly seen in children.
• Bacteremia (def) occurs in 25% - 30% of individuals with pneumococcal pneumonia; in more than 80% with meningitis. Endocarditis (def) can be a complication in both those with normal and previously damaged heart valves.

From Pneumococcal Infections , by Christian P Sinave, MD, FRCPC, Associate Professor, Department of Medical Microbiology and Infectious Diseases, University of Sherbrooke, Canada and Christian P Sinave, MD, FRCPC, a member of the American Society for Microbiology, and Canadian Infectious Disease Society.