pneumoniae (the Pneumococcus)
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).
- Colonizes the nasopharynx in humans.
- 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.
- 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.
- An opportunistic bacterium causing
otitis media (def)
and sinusitis, typically following a viral infection of the upper respiratory
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..
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.
(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
- The capsule
serves as the major virulence factor, enabling the pneumococcus to resist
- 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 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
*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.
Kaiser's Microbiology Home Page
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Updated: January 27, 2005
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