Definitive identification can be made with either biochemical tests or thecapsular swelling (quellung) reaction

The statement you're referring to highlights methods used in microbiology for the identification of certain bacteria, particularly those that possess a polysaccharide capsule, such as Streptococcus pneumoniae.

1. Biochemical Tests: These tests involve using specific substrates to see if a bacterium can utilize them or produce certain enzymes. Biochemical tests can help distinguish between different bacterial species based on their metabolic capabilities. For instance, tests like catalase, oxidase, fermentation of sugars, and enzyme production can yield significant information about the organism's identity.

2. Quellung Reaction: The Quellung reaction is a specific test used to identify encapsulated bacteria. In this test, a specific antiserum (antibody) is mixed with the bacterial sample. If the bacteria possess the specific capsular polysaccharide, the antiserum will bind to it, causing the capsule to swell and making the bacteria appear larger under the microscope. This swelling is a definitive indicator of the presence of certain capsulated bacteria.

Both methods provide vital information for accurate bacterial identification, with biochemical tests being broad and versatile, while the Quellung reaction allows for more specific identification of encapsulated strains.