The Quellung (Neufeld's) reaction is used to identify and serotype encapsulated bacteria. What is the correct principle of this reaction?
- A The reaction measures complement consumption by encapsulated bacteria — a decrease in CH50 confirms capsular type
- B Type-specific antibody binds to the polysaccharide capsule, causing a visible apparent swelling (actually capsule becomes more refractive/sharply demarcated) visible under light microscopy when methylene blue is added as counterstain ✓
- C The reaction exploits the inhibition of crystal violet uptake by capsular polysaccharide, producing a negative staining effect on India ink preparations
- D Type-specific antibody causes true physical capsular swelling via osmotic water influx into the capsule matrix
Explanation
The Quellung (German for 'swelling') reaction — also called Neufeld's capsular reaction — does not involve true capsular swelling. When type-specific anticapsular antibody binds to the polysaccharide capsule of bacteria (classically Streptococcus pneumoniae, also Klebsiella, H. influenzae, Neisseria), the antibody-capsule complex becomes more refractive and optically distinct, appearing as a clearly demarcated halo around the bacterium under the microscope (especially with methylene blue counterstain). This apparent swelling is an optical phenomenon due to altered light refraction at the antibody-bound capsule boundary, not true physical enlargement. The reaction is used for serotyping and identification. India ink is a negative staining technique that visualizes the capsule space, not the Quellung reaction.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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Written and medically reviewed by the StethoPrep medical team.