What is the quellung test used to identify?

The quellung test is specifically used to identify Streptococcus pneumoniae, which is the correct answer. This test relies on the serological reaction that occurs when specific antisera are mixed with bacterial capsules. When S. pneumoniae is present, the polysaccharide capsule will swell due to the binding of the antiserum, making the bacteria easier to visualize under a microscope.

This method is particularly useful for confirming the diagnosis of pneumococcal infections, as S. pneumoniae is known for its virulent polysaccharide capsule that plays a key role in its pathogenicity. The quellung reaction is a classic technique and remains a reliable way to identify this pathogen in the laboratory, especially in cases of pneumonia, meningitis, or other infections caused by S. pneumoniae.

Other bacterial species mentioned in the options, such as S. pyogenes, S. agalactiae, and S. sanguis, do not exhibit a swelling reaction in the quellung test, which is specifically characteristic of the polysaccharide capsule of S. pneumoniae.