Streptococcus pneumoniae

Streptococcus pneumoniae (the Pneumococcus)

Organism

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.

Pathogenicity

The capsule serves as the major virulence factor, enabling the pneumococcus to resist phagocytic engulfment.
Peptidoglycan fragments and teichoic acids from its gram-positive cell wall can lead to excessive cytokine production and a massive inflammatory response. Inflammation in lung tissue results in the alveoli (def) becoming filled with plasma (def) which, in turn, prevents gas exchange.
Adheres to epithelial cells of the respiratory mucosa by way of surface protein adhesins (def).
Produces secretory IgA (def) protease to break down protective antibodies in the mucus.
Produces a type II toxin (def) called pneumolysin that binds cholesterol in host cell membranes creating pores that can kill ciliated epithelial cells and phagocytes. Pneumolysin also prevents the oxidative burst in phagocytes that produces the microbicidal oxygen free radicals used to kill microbes during phagocytosis.
Produces phosphorylcholine in its cell wall that functions as an invasin (def) enabling the bacterium to enter host cells where it can resist phagocytosis as well as pass into the blood and central nervous system.

Treatment

Treatment includes cephalosporins, macrolides, vancomycin, or penicillins in strains still susceptible to penicillins* (see antibiotic table).
Active immunization (def) with a heptavalent pneumococcal conjugate vaccine in children or a pneumococcal conjugate vaccine containing capsular material from the 23 most common strains of pneumococci causing infection in the U.S. given to those at high risk for pneumococcal pneumonia.

*Drugs may change with time.

For a more detailed article on Streptococcus pneumoniae, see 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.