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Tag words: Streptococcus pneumoniae, S pneumoniae, pneumococcus, lobar pneumonia, pneumococcal pneumonia, otitis media, meningitis, pneumococcal vaccine.

Streptococcus pneumoniae

Kingdom: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Lactobacillales
Family: Streptococcaceae
Genus: Streptococcus
Species: S. pneumoniae

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interests include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Streptococcus pneumoniae (page 1)

(This chapter has 4 pages)

© Kenneth Todar, PhD


Streptococcus pneumoniae
is a normal inhabitant of the human upper respiratory tract. The bacterium can cause pneumonia, usually of the lobar type, paranasal sinusitis and otitis media, or meningitis, which is usually secondary to one of the former infections. It also causes osteomyelitis, septic arthritis, endocarditis, peritonitis, cellulitis and brain abscesses. Streptococcus pneumoniae is currently the leading cause of invasive bacterial disease in children and the elderly. Streptococcus pneumoniae is known in medical microbiology as the pneumococcus, referring to its morphology and its consistent involvement in pneumococcal pneumonia.

Pneumonia is a disease of the lung that is caused by a variety of bacteria including Streptococcus, Staphylococcus, Pseudomonas, Haemophilus, Chlamydia and Mycoplasma, several viruses, and certain fungi and protozoans. The disease may be divided into two forms, bronchial pneumonia and lobar pneumonia. Bronchial pneumonia is most prevalent in infants, young children and aged adults. It is caused by various bacteria, including Streptococcus pneumoniae. Bronchial pneumonia involves the alveoli contiguous to the larger bronchioles of the bronchial tree. Lobar pneumonia is more prone to occur in younger adults. A majority (more than 80%) of the cases of lobar pneumonia are caused by Streptococcus pneumoniae. Lobar pneumonia involves all of a single lobe of the lungs (although more than one lobe may be involved), wherein the entire area of involvement tends to become a consolidated mass, in contrast to the spongy texture of normal lung tissue.


Streptococcus pneumoniae cells are Gram-positive, lancet-shaped cocci (elongated cocci with a slightly pointed outer curvature). Usually, they are seen as pairs of cocci (diplococci), but they may also occur singly and in short chains. When cultured on blood agar, they are alpha hemolytic. Individual cells are between 0.5 and 1.25 micrometers in diameter. They do not form spores, and they are nonmotile. Like other streptococci, they lack catalase and ferment glucose to lactic acid. Unlike other streptococci, they do not display an M protein, they hydrolyze inulin, and their cell wall composition is characteristic both in terms of their peptidoglycan and their teichoic acid.

Gram Stain of a film of sputum from a case of lobar pneumonia. CDC.


Streptococcus pneumoniae is a fastidious bacterium, growing best in 5% carbon dioxide. Nearly 20% of fresh clinical isolates require fully anaerobic conditions. In all cases, growth requires a source of catalase (e.g. blood) to neutralize the large amount of hydrogen peroxide produced by the bacteria.  In complex media containing blood, at 37°C,  the bacterium has a doubling time of 20-30 minutes.

On agar, pneumococci grow as  glistening colonies, about 1 mm in diameter. Two serotypes, types 3 and 37, are mucoid. Pneumococci spontaneously undergo a genetically determined, phase variation from opaque to transparent colonies at a rate of 1 in 105 . The transparent colony type is adapted to colonization of the nasopharynx, whereas the opaque variant is suited for survival in blood. The chemical basis for the difference in colony appearance is not known, but significant difference in surface protein expression between the two types has been shown.

Streptococcus pneumoniae is a fermentative aerotolerant anaerobe. It is usually cultured in media that contain blood. On blood agar, colonies characteristically produce a zone of alpha (green) hemolysis, which differentiates S. pneumoniae from the group A (beta hemolytic) streptococcus, but not from commensal alpha hemolytic (viridans) streptococci which are co-inhabitants of the upper respiratory tract. Special tests such as inulin fermentation, bile solubility, and optochin (an antibiotic) sensitivity must be routinely employed to differentiate the pneumococcus from Streptococcus viridans.

Streptococcus pneumoniae Gram-stain of blood broth culture. CDC.

Streptococcus pneumoniae is a very fragile bacterium and contains within itself the enzymatic ability to disrupt and to disintegrate the cells. The enzyme responsible is called an autolysin. The physiological role of this autolysin is to cause the culture to undergo a characteristic autolysis that kills the entire culture when grown to stationary phase. Virtually all clinical isolates of pneumococci harbor this autolysin and undergo lysis usually beginning between 18-24 hours after initiation of growth under optimal conditions. Autolysis is consistent with changes in colony morphology. Colonies initially appear with a plateau-type morphology, then start to collapse in the centers when autolysis begins.


The minimum criteria for identification and distinction of pneumococci from other streptococci are bile or optochin sensitivity, Gram-positive staining, and hemolytic activity. Pneumococci cause alpha hemolysis on agar containing horse, human, rabbit and sheep erythrocytes. Under anaerobic conditions they switch to beta hemolysis caused by an oxygen-labile hemolysin. Typically, pneumococci form a 16-mm zone of inhibition around a 5 mg optochin disc, and undergo lysis by bile salts (e.g. deoxycholate). Addition of a few drops of 10% deoxycholate at 37°C lyses the entire culture in minutes. The ability of deoxycholate to dissolve the cell wall, depends upon the presence of the autolytic enzyme, LytA. Virtually all clinical isolates of pneumococci harbor the autolysin and undergo deoxycholate lysis.

Streptococcus pneumoniae A mucoid strain on blood agar showing alpha hemolysis (green zone surrounding colonies). Note the zone of inhibition around a filter paper disc impregnated with optochin. Viridans streptococci are not inhibited by optochin.  


The quellung reaction (swelling reaction) forms the basis of serotyping and relies on the swelling of the capsule upon binding of homologous antibody. The test consists of mixing a loopful of colony with equal quantity of specific antiserum and then examining microscopically at 1000X for capsular swelling. Although generally highly specific, cross-reactivity has been observed between capsular types 2 and 5, 3 and 8, 7 and 18, 13 and 30, and with E. coli, Klebsiella, H. influenzae Type b, and certain viridans streptococci.

Streptococcus pneumoniae Quellung (capsular swelling) reaction can be used to demonstrate the presence of a specific capsular type of the bacterium.

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Kenneth Todar is an emeritus lecturer at University of Wisconsin-Madison. He has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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