What type of test is used for serotyping when capsules are present?

Serotyping by swelling is specifically utilized when capsules are present because this method allows for the identification of encapsulated bacteria. Capsules often play a crucial role in the virulence and pathogenicity of bacteria, making their detection and characterization important for understanding the organism's behavior and potential impact on public health.

In this testing method, the bacteria are exposed to specific antisera that can recognize the capsular antigens. If the specific antigens are present, a visible swelling occurs around the bacteria, indicating a positive reaction. This is critical for serotyping, as it helps differentiate between strains based on their capsule types, aiding in identifying bacterial species and understanding their epidemiology.

While the other types of tests listed have their uses in immunology and microbiology, they do not specifically target capsular antigens or rely on the detection of capsule-induced reactions in the same direct manner as serotyping by swelling. This specificity is what makes serotyping by swelling the most appropriate choice when dealing with encapsulated bacteria.