The Quellung (capsular swelling) reaction is classically used for the typing of which encapsulated pathogen in clinical microbiology?

Introduction / Context:
The Quellung reaction, meaning “swelling” in German, is an antibody-mediated capsular swelling test used historically to serotype encapsulated bacteria. It is particularly associated with identifying serotypes of Streptococcus pneumoniae for epidemiology and vaccine studies.

Given Data / Assumptions:

• Test principle: capsular polysaccharide + specific antisera → visible halo enlargement under microscopy.
• Primary clinical application: pneumococcal serotyping.

Concept / Approach:
When specific anti-capsular antibodies bind the polysaccharide capsule of S. pneumoniae, the capsule refracts light more and appears expanded. Although capsular reactions can be observed with some other organisms, the classic, routine application is for pneumococci.

Step-by-Step Solution:

Verification / Alternative check:
Reference methods for pneumococcal serogrouping/serotyping include Quellung and latex agglutination with type-specific antisera.

Why Other Options Are Wrong:

• Klebsiella pneumoniae: Encapsulated, but Quellung is not the classic typing method.
• Both/None/Neisseria only: Do not reflect the standard, textbook application.

Common Pitfalls:

• Assuming any encapsulated bacterium is routinely typed by Quellung; pneumococcus remains the archetypal use.

Final Answer:
Streptococcus pneumoniae (pneumococcus)