ABSTRACT
The bacterium Haemophilus influenzae was initially described by Pfeiffer who presumed it to be the causative agent for the 1889-1891 influenza pandemic. The organism is a gram-negative aerobic bacillus that requires both X factor (hemin) and V factor (nicotinamide adenine dinucleotide). X factor is released into the medium by red blood cells. Although V factor is also present in red blood cells, it is not released into the medium unless the cells are lysed. Consequently, H. influenzae will not grow on blood agar except as satellite colonies around other colonies that produce hemolysis.
