Streptococcus pneumoniae using the Quellung reaction with multitype (or polyvalent) antiserum — an important classical microbiological method.
🔬 Quellung Reaction for Streptococcus pneumoniae:
Definition:
The Quellung reaction (from German "quellen" = to swell) is a microscopic capsule swelling test used to identify encapsulated bacteria, especially Streptococcus pneumoniae.
🧪 Principle:
When specific antisera (multitype antiserum against many S. pneumoniae capsular types) are added to a sample containing S. pneumoniae, the capsular polysaccharide reacts with the antibodies and becomes more refractile and swollen, making it more visible under a microscope.
✅ Procedure:
A bacterial suspension (e.g., from sputum or CSF) is mixed with multitype pneumococcal antiserum on a glass slide.
A coverslip is placed over the drop.
The slide is examined under a phase contrast or bright-field microscope.
A positive reaction shows clear capsule swelling around the diplococci — known as the "quellung" effect.
🔎 Interpretation:
Positive Quellung Reaction: Swollen, sharply outlined capsule — confirms S. pneumoniae.
Negative: No capsule swelling.
🦠 Key Points:
S. pneumoniae is encapsulated, Gram-positive diplococcus.
Over 90 serotypes exist — hence, multitype (polyvalent) antisera are used for general detection.
The Quellung reaction is specific and confirmatory, though largely replaced today by PCR and antigen detection tests.
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