In this case the most likely to be Bordetella pertussis. This is a gram-negative, aerobic bacterium that grows well in glycerin-potato-blood agar of Bordet and Gengou and shows a slower growth pattern, usually requiring from three to five days after incubation. Additionally, the colonies are beta-hemolytic, small and dome-shaped, possessing a gray metallic-look, sometimes similar to mercury droplets.
