Streptococcus pneumoniae (pneumococcus): Disease, Properties, Pathogenesis and Laboratory diagnosis

Streptococcus pneumoniae is a Gram positive cocci, typically lancet shaped and occurs singly, in pairs (diplococci) or in short chains.

Streptococcus pneumoniae in Gram Staining
Streptococcus pneumoniae in Gram Staining

Main diseases caused by Streptococcus pneumoniae:

  1. Bacterial pneumonia: Most common bacterial cause, especially in infants and elderly. Pneumococcus in alveoli stimulate release of fluid, red and white cells producing “rusty sputum”.
  2. Meningitis: one of the most common cause of meningitis (Others are Haeomophilus influenzae, and Neisseria meningitidis). CSF generally has very high WBC and low glucose.
  3. Otitis media: common in children.
  4. Sinusitis: common in children.
  5. Bronchitis
  6. Bacteremia and other infectious processes

Virulence factors and Pathogenesis of Streptococcus pneumoniae

Types of disease caused by Streptococcus pneumoniae
Types of disease caused by Streptococcus pneumoniae
  1. Polysaccharide capsule : It is the major virulence factor of Streptococcus pneumoniae. (Antiphagocytic in nature). Antibody to the polysaccharide capsule (>80 serotypes) provides type-specific immunity.
  2. Peptidoglycan/teichoic acids are highly inflammatory in central nervous system.
  3. IgA protease: Helps in colonization
  4. Teichoic acids: Helps in adherence
  5. Pneumolysin O: Hemolysin/Cytolysin
    Damages respiratory epithelium (hemolysin is similar to streptolysin O, which damages eukaryotic cells).
    Inhibits leukocyte respiratory burst and inhibits classical complement fixation.

Reservoirs: Harmlessly inhabit the upper respiratory tract of humans.

Transmission: Respiratory droplets, not considered highly communicable. Often colonizes without causing infection.

Colony characteristics: Alpha hemolytic (partial discoloration around the bacterial colony in blood agar), mucoid colony (if organism possesses polysaccharide capsule).

Rapid test: Catalase test: negative (no gas bubble formed when colonies of Streptococcus pneumoniae is emulsified in hydrogen peroxide solution)

Biochemical/serological tests to identify Streptococcus pneumoniae:

  1. Presumptive test: Susceptible (sensitive) to Optochin (Ethyl hydrocupreine hydrochloride): When a filter paper disk impregnated with optochin is plated on blood agar plate previously streaked with a suspect S. pneumoniae and incubated in 35oC for 18-24 hours, S. pneumoniae produce zone of inhibition (sensitive to optochin).

    Alpha hemolysis in Blood Agar by Streptococcus pneumoniae, See Zone of inhibition around Optochin disk
    Alpha hemolysis in Blood Agar by Streptococcus pneumoniae, observe zone of inhibition around Optochin disk
  2. Confirmatory test: Bile solubility test positive
    This test is based on the ability of bile salts to lyse Streptococcus pneumoniae colonies.
  3. Quellung Reaction: Positive
    Streptococcus pneumoniae shows Quellung positive reaction
    Streptococcus pneumoniae shows quellung positive reaction

    When Streptococcus pneumoniae of certain types are mixed with specific antipolysaccharide serum of the same type or with polyvalent antiserum on a microscope slide, the capsule swells markedly and organism agglutinate by cross linking of the antibodies.

Antimicrobial Resistance

Initially, penicillin was the drug of choice to treat S. pneumoniae infections as all S. pneumoniae isolates were exquisitely susceptible to penicillin but  now drug-resistant S.pneumoniae (DRSP) possess serious threat to global health as we are observing an alarming worldwide increase in the incidence.


Antimicrobial sensitivity testing of Streptococcus pneumoniae
Antimicrobial sensitivity testing of Streptococcus pneumoniae

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