Streptococcus pneumoniae (pneumococcus): Disease, Properties

By Acharya Tankeshwar •  Updated: 05/04/22 •  3 min read

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 the elderly. Pneumococci in alveoli stimulate the release of fluid, red and white cells producing “rusty sputum”.
  2. Meningitis: one of the most common causes of meningitis (Others are Haemophilus 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 a blood agar plate previously streaked with a suspect S. pneumoniae and incubated at 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 a 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

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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8 responses to “Staphylococcus vs. Micrococcus”

  1. Mohit says:

    How to differentiate micrococci on gram stain from staphylococci

    • Tankeshwar Acharya says:

      Its difficult to differentiate Micrococci and Staphylococci in Gram stain as both are Gram positive cocci and may appear in tetrads and clusters.Experienced Microbiologist take accounts of size (Micrococci is slightly bigger than Staphylococci), and arrangements (in Micrococcus you mostly observed tetrads instead of clusters).

  2. Rauf says:

    diagnosis and prevention of micrococcus. send to my email at

  3. shivani says:

    Sir can u plz tell me for staph aureus tube coagulase is necessary slide agglutination is not sufficient

  4. Poonam says:

    Sir can you plz tell me the biochemical test for staphylococcus and ways of differentiating it from miicrococcus.

    • Esther says:

      I came across large gram positive cocci in pairs, tetras and in 8s, did a coagulase test and it came out positive but sensitive to bacitracin.
      How can I further tell if its micrococcus or staphylococcus since its coagulase positive or is there any specie of micrococcus that is coagulase positive?
      Thank you

  5. Syafiqah says:

    How to differentiate micrococcus and staph on mannitol salt agar?

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