What does a positive Quellung test indicate?

A positive Quellung test indicates capsular swelling due to an antigen-antibody reaction. This test is primarily used for the identification of certain bacterial pathogens, particularly encapsulated organisms like Streptococcus pneumoniae. When the polysaccharide capsule of the bacteria interacts with specific antibodies, it causes a visible swelling of the capsule, making the organism more easily identifiable under a microscope.

This reaction is critical because the presence of a capsule is a key virulence factor for many pathogens, providing them with protection against phagocytosis and aiding in their ability to cause disease. Therefore, observing this swelling means that specific antibodies have bound to the capsule, confirming the identity of the bacteria based on the capsule's serotype.

The context of the other options emphasizes that while some may relate to identification or characteristics of bacteria, they do not pertain to the specific antigen-antibody interaction that the Quellung test aims to reveal.