A sputum culture on egg-based medium yields smooth brown colonies after 3 weeks; which organism is most likely?

A sputum culture on egg-based medium yields smooth brown colonies after 3 weeks; which organism is most likely?

• A. Mycobacterium bovis
• B. Mycobacterium gordonae
• C. MAC (Mycobacterium avium complex)
• D. Mycobacterium tuberculosis

Answer: (C)

Growth on egg-based mycobacterial media follows specific patterns of colony texture, pigmentation, and how long it takes to appear. The finding of smooth, brown pigmented colonies after about three weeks points to a slow-growing mycobacterium that produces pigment in this medium. Mycobacterium avium complex is known for forming smooth, creamy to pigmented colonies on Löwenstein-Jensen–type egg media and often requires several weeks to become visible, which matches this observation well. In contrast, the classic tuberculosis complex organisms (Mycobacterium tuberculosis and Mycobacterium bovis) typically produce buff, dry, rough, nonpigmented colonies on egg-based media. Mycobacterium gordonae is usually nonpathogenic and often shows less consistent pigment patterns, not the smooth brown pigment described here. Therefore, the described culture morphology most closely fits the Mycobacterium avium complex.

Growth on egg-based mycobacterial media follows specific patterns of colony texture, pigmentation, and how long it takes to appear. The finding of smooth, brown pigmented colonies after about three weeks points to a slow-growing mycobacterium that produces pigment in this medium. Mycobacterium avium complex is known for forming smooth, creamy to pigmented colonies on Löwenstein-Jensen–type egg media and often requires several weeks to become visible, which matches this observation well.

In contrast, the classic tuberculosis complex organisms (Mycobacterium tuberculosis and Mycobacterium bovis) typically produce buff, dry, rough, nonpigmented colonies on egg-based media. Mycobacterium gordonae is usually nonpathogenic and often shows less consistent pigment patterns, not the smooth brown pigment described here. Therefore, the described culture morphology most closely fits the Mycobacterium avium complex.