Question
Satellitism.
Answer
Satellitism is a phenomenon on culture plates in which colonies of a fastidious organism grow larger and more prominent specifically around/near colonies of another organism that supplies an essential growth factor it cannot itself synthesize — colony size progressively diminishes with distance from the “helper” colony.
Classic example: Haemophilus influenzae requires two growth factors — factor V (NAD) and factor X (haemin) — that are not adequately available together on ordinary blood agar (red cells supply factor X, but NADase present in blood inactivates factor V). When H. influenzae is plated on blood agar alongside a streak of Staphylococcus aureus, the staphylococcal colonies lyse red cells and release/synthesize factor V (NAD) into the surrounding medium; H. influenzae colonies growing close to the S. aureus streak therefore appear larger — the satellite phenomenon — while colonies farther away remain tiny or fail to grow altogether.
Diagnostic use: the satellite test provides a rapid, low-cost presumptive method for identifying H. influenzae in the laboratory by exploiting this dual growth-factor dependency, and can help differentiate it from other fastidious Gram-negative coccobacilli that lack the same requirement.