Question
Satellitism.
Answer
Satellitism is a phenomenon seen 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 synthesize — colony size progressively diminishes with increasing distance from the “helper” colony.
Classic example: Haemophilus influenzae requires two growth factors — factor V (NAD) and factor X (haemin) — which are not adequately available on ordinary blood agar (RBCs contain factor X but the NADase in blood inactivates factor V). When H. influenzae is plated on blood agar alongside Staphylococcus aureus, the staphylococcal colonies lyse red cells and release/synthesize factor V (NAD) into the surrounding medium (factor X is already present from the lysed RBCs); H. influenzae colonies growing near the S. aureus streak therefore appear larger, exhibiting the satellite phenomenon, while distant colonies remain tiny or fail to grow.
Diagnostic use: the satellitism/satellite test is used as a rapid presumptive method to identify H. influenzae in the laboratory, exploiting this X and V factor dependency, and can also help distinguish it from other Gram-negative coccobacilli that do not show the same growth-factor requirement.