Streptococcus pneumoniae (pneumococcus): Disease, Properties

Last updated on June 25th, 2021

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

Main diseases caused by Streptococcus pneumoniae:

Streptococcus pneumoniae in Gram Staining
Streptococcus pneumoniae in Gram Staining
  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 causes 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 the 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:

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

Presumptive test: Susceptible to Optochin (ethylhydrocupreine 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 35°C for 18-24 hours, S. pneumoniae produce a zone of inhibition (sensitive to optochin).

S. pneumoniae shows Quellung positive reaction
Streptococcus pneumoniae shows quellung positive reaction

Confirmatory test: Bile solubility test positive: This test is based on the ability of bile salts to lyse Streptococcus pneumoniae colonies.

Quellung Reaction: Positive
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 the organism agglutinates 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
About Acharya Tankeshwar 476 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.


  1. Hello sir

    Information provided by you is really very helpful.

    Please guide me for detailed procedure for preparing sheep blood agar plates for culturing streptococcus pneumoniae

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