After 72 hours of anaerobic incubation, small olive-green to black colonies are seen; a Gram stain reveals gram-positive cocci. Which organism is most likely?

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

Answer: (B)

Pigment production by anaerobic gram-positive cocci is a classic clue. The combination of anaerobic growth, small colonies that appear olive-green to black, and a Gram stain showing gram-positive cocci points to a pigment-producing anaerobic coccus. Peptococcus niger is well known for forming dark pigmented colonies under anaerobic conditions, giving that olive-green to black appearance as it oxidizes pigment with time. Other organisms listed are either not pigmented or not gram-positive cocci (for example, Veillonella parvula is Gram-negative), so they don’t fit the observed characteristics. Thus, Peptococcus niger best explains the findings.