Review Question - QID 101467

The quellung reaction is a test which uses antibodies to test for the presence of a bacterial capsule. The bacterial capsule helps the organism prevent phagocytosis and is therefore necessary for infection.

S. pneumoniae is gram-positive, lancet-shaped diplococcus that typically displays alpha-hemolysis, optochin sensitivity, and bile-solubility. Notably, it also displays a positive quellung reaction, owing to its capsule. When incubated together with a specific anti-capsule antibody, the bacterial capsule (if present) swells and can be visualized under a microscope as a positive quellung reaction (see Illustration A). In S. pneumoniae, the capsule is necessary for pathogenesis and an immune response to this bacteria generally contains antibodies against this target. The antibodies facilitate opsonization and eventual phagocytosis by macrophages. S. pneumoniae is the most common cause of meningitis, pneumonia, sinusitis, and otitis media in children.

According to Dilwort et al., direct quellung testing on sputum smears was a 95% sensitive test for predicting pneumococcal isolation by sputum culture. For this reason, it was recommended by this study for general use for the detection of pathogenic pneumococcal pneumonia in patients who had a consistent clinical picture.

Bamberger et al. review bacterial meningitis, and state that dexamethasone initiated with antimicrobial therapy demonstrated an improved mortality and morbidity rate. Additionally, they found that 30% of pneumococcal strains were not susceptible to penicillin. Pneumococcus is one of the most common cause of meningitis in the US.

Illustration A depicts a positive quellung reaction, as described above.

Incorrect Answers:
Answer 1: S. pneumoniae is bile soluble and changing this characteristic would not likely produce a difference in pathogenesis.
Answer 2: S. pneumoniae is optochin sensitive and changing this characteristic would not likely produce a difference in pathogenesis.
Answer 4: S. pneumoniae is alpha-hemolytic, changing this property would likely not have as drastic of consequences as removal of the bacterial capsule to the pathogenesis of the organism.
Answer 5: S. pneumoniae is a non-motile organism and creating a motile strain would likely have no effect on pathogenesis.