Which organism is characterized by large, α-hemolytic colonies on blood agar?

The organism characterized by large, α-hemolytic colonies on blood agar is Listeria spp. Listeria monocytogenes, in particular, is known for forming distinct colonies on blood agar that demonstrate α-hemolysis, meaning it produces a green discoloration around the colonies due to partial lysis of red blood cells. This hemolytic behavior is a key identifying trait of Listeria spp.

In contrast, Pseudomonas spp. are typically known for producing large, β-hemolytic colonies, and they also have a characteristic grape-like odor and a distinct pigmentation, which varies with the species. Bacillus spp. are known for their ability to form spores and can exhibit different hemolytic patterns that do not specifically categorize them as α-hemolytic. Corynebacterium spp., such as C. diphtheriae, may show some hemolytic activity but are generally not recognized for large α-hemolytic colonies. Thus, the unique trait of Listeria spp. on blood agar supports its identification as the organism described by the question.