Which of the following Aspergillus species produce black coloured colonies on sabaraud’s dextrose agar?
**Core Concept**
Aspergillus species exhibit varying colony morphologies on different agar media, which is crucial for their identification in clinical and environmental samples. The production of black-colored colonies on Sabouraud's dextrose agar is a characteristic feature of certain Aspergillus species.
**Why the Correct Answer is Right**
Aspergillus niger produces black-colored colonies on Sabouraud's dextrose agar due to the presence of melanin, a pigment that is synthesized through the action of the polyketide synthase (PKS) enzyme. This enzyme catalyzes the production of melanin from a variety of precursors, including phenolic compounds. The production of melanin is a key factor in the pathogenicity of A. niger, as it allows the fungus to protect itself from oxidative stress and environmental damage.
**Why Each Wrong Option is Incorrect**
**Option B:** Aspergillus fumigatus produces greenish or yellowish colonies on Sabouraud's dextrose agar, not black. This species is more commonly associated with human disease, particularly allergic bronchopulmonary aspergillosis.
**Option C:** Aspergillus terreus produces a range of colony colors, including yellow, green, and brown, but not black. This species is often associated with infections in immunocompromised individuals.
**Option D:** Aspergillus flavus produces yellow or greenish colonies on Sabouraud's dextrose agar, and is known for producing aflatoxins, which are toxic to humans and animals.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the production of melanin is a key characteristic of A. niger, and can be used to distinguish it from other Aspergillus species.
**Correct Answer:** B. Aspergillus fumigatus.