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:
- 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”.
- 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.
- Otitis media: common in children.
- Sinusitis: common in children.
- Bacteremia and other infectious processes
Virulence factors and Pathogenesis of Streptococcus pneumoniae
- 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.
- Peptidoglycan/teichoic acids are highly inflammatory in the central nervous system.
- IgA protease: Helps in colonization
- Teichoic acids: Helps in adherence
- 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.
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:
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).
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.
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.