Agram-negative coccobacillus produces small gray colonies surrounded by a zone of hemolysis on potato-blood-glycerol agar. the organism also grew on chocolate agar, but not on sheep blood agar. this is most likely to be which organism

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. 

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