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How to identify Streptococcus pneumoniae?

 Streptococcus pneumoniae (pneumococci) are a part of the normal nasopharyngeal and oropharyngeal flora. It is an important etiological agent of upper and lower respiratory tract infections (URTI and LRTI) , bacteremia and septicemia. Streptococcus pneumoniae   is also associated with otitis media, sinusitis, meningitis and endocarditis.

Lanceolate diplococci. Image source: microbiologyinpictures.com

Pneumococci are Gram positive lancet shaped diplococci (intracellularly or extracellularly), non-motile, and encapsulated. They occur in pairs with the broader end opposed, hence called Gram positive diplococci. S. pneumoniae is a fastidious bacterium, which grows best at  at 35-37°C with ~5% CO2 (or in a candle-jar).

Laboratory diagnosis

Laboratory diagnosis of Streptococcus pneumoniae infection is based on finding characteristics shape of the organism in the sputum, characteristic colony morphology, biochemical reactions, susceptibility to certain diagnostic discs and latex agglutination test.

Culture and  identification during suspected Streptococcus pneumoniae infection

Flow chart for identification and characterization of a S. pneumoniae isolate
  1. Microscopy and Staining 

2. Culture and Sensitivity 

   Inoculate sample onto blood agar and chocolate agar plate.

Colony morphology

Identification of Streptococcus pneumoniae by biochemical reactions.

      A. Optochin test (6 mm disc with 5µg).

·         Observe the zone of growth inhibition around the disc and interpret as:

B.  Bile solubility test.

 

Reading

Note:  This test can also be done directly onto the colony and the colony is lysed by addition of bile solution.

Interpretation of Optochin and Bile solubility test

Antimicrobial susceptibility

Reporting of results: Streptococcus pneumoniae isolated and resistance patterns with tested antibiotics

3. Detection of the antigen

C-carbohydrate antigen of the Streptococcus pneumoniae can be detected in the urine (Read:Pneumococcal Urinary Antigen Testing (UAT): Principle, Procedure and Results ) for the diagnosis of pneumonia and in CSF for the diagnosis of pneumococcal meningitis.