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◉ Definition
Most strains of Haemophilus spp do not grow on 5% sheep blood agar, which contains hemin (factor X) but lacks NAD (factor V).
Haemophilus influenzae uses factor X to produce essential respiratory enzymes like cytochromes, catalases and peroxidases. Factor V is used as an electron carrier in the body's oxidation-reduction system.
Staphylococcus aureus produces NAD as a metabolic by-product when growing in culture medium containing blood. Therefore, Haemophilus spp can grow on sheep blood agar very close to colonies of Staphylococcus aureus (because it produces the NAD factor), this phenomenon is known as satellitism.
◉ Procedure
- Mix suspected Haemophilus colonies in 2ml of sterile saline or sterile peptone water.
- Using a sterile swab, inoculate the organism suspension onto a nutrient agar plate and a blood agar plate.
- Trace a pure culture of S. aureus on each of the inoculated plates.
- Incubate the two dishes in an atmosphere enriched with carbon dioxide at 37°C for 18 hours.
- Examine cultures for growth and satellite colonies.
◉ Interpretation of the result
- Haemophilus influenzae : Shows growth on the blood agar plate, but not on the nutrient agar plate, and colonies near the S. aureus growth column are larger than those away from it.
-If satellite colonies are present on both dishes, the organism is probably a species of Haemophilus that only requires factor V (NAD) such as H. parainfluenzae.
