After 72 hours of anaerobic incubation, small olive-green to black colonies are seen. A Gram stain reveals Gram-positive cocci. What is the most likely identification?

• A. Fingoldia magna
• B. Peptococcus niger
• C. Peptostreptococcus anaerobius
• D. Veillonella parvula

Answer: (B)

Pigmented colonies produced by anaerobic Gram-positive cocci point to Peptococcus niger. This organism is classic for forming dark, sometimes olive-green to black, pigmented colonies when grown anaerobically, especially after extended incubation. The Gram stain showing Gram-positive cocci fits Peptococcus, whereas Veillonella parvula is Gram-negative, so it wouldn’t match the gram stain. Peptostreptococcus anaerobius is Gram-positive cocci as well, but it typically does not produce pigmented, dark colonies. Fingoldia magna is not the characteristic pigmented clinical isolate here. Putting the growth pattern together with the Gram-positive cocci result best identifies Peptococcus niger.