Normal Flora of the Nasopharynx
Streptococcus
Members of the genus Streptococcus are commonly found as resident flora in the nose and throat of humans. S. mutans, S. gordonii, and S. salivarius are examples of resident mouth flora. Group A Strep such as S. pyogenes (one are found in approximately 5-15% of normal individuals as part of the normal throat flora.
Streptococcus species are cultured on blood agar plates, and classified based on hemolytic patterns.
Neisseria
Members of the genus Neisseria are commonly found as part of the normal flora in the throat and mouth, including several saprophytic (non-pathogenic) species of Neisseria. In addition, the pathogen Neisseria meningitidis is found in approximately 20-40% of young adults as a normal resident of the nasopharynx.
Neisseria are fastidious organisms that are cultured on chocolate agar, which contains lysed red blood cells. Following incubation, Neisseria are distinguished from other nasopharyngeal flora by performing an oxidase test. To perform the test, a drop of oxidase reagent is added to the growth on the plates. A purple color indicates the organism is oxidase-positive and belongs to the genus Neisseria.
(Note: The oxidase test is testing for the presence of cytochrome c in the electron transport chain.)