What is responsible for the satellite growth of Haemophilus spp. around colonies of Staphylococcus and Neisseria spp. on sheep blood agar?

The satellite growth of Haemophilus species around colonies of Staphylococcus and Neisseria species on sheep blood agar is primarily due to the presence of NAD (nicotinamide adenine dinucleotide). Haemophilus species are fastidious organisms that require specific growth factors to thrive, two of which are hemin (factor X) and nicotinamide adenine dinucleotide (NAD or factor V).

In the case of satellite growth, the Staphylococcus and Neisseria species produce NAD as a metabolic byproduct, which diffuses into the surrounding medium. This creates an environment conducive to the growth of Haemophilus species, despite the absence of NAD in the sheep blood agar itself. The closer proximity of Haemophilus to the colonies of Staphylococcus or Neisseria allows it to utilize the NAD secreted by these bacteria, leading to the characteristic satellite growth pattern. This principle illustrates the interdependence of bacterial species in a mixed-culture environment, highlighting how certain bacteria can facilitate the growth of others by providing essential nutrients.