In an anaerobic culture, olive-green to black colonies consisting of gram-positive cocci most likely represent which organism?

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

Answer: (B)

An anaerobic culture producing olive-green to black colonies composed of gram-positive cocci points to a pigmented anaerobe whose colonies darken as they grow. Peptococcus niger is a classic example of this pattern: it’s an obligate anaerobe, a gram-positive coccus, and it forms dark pigmented colonies on anaerobic media. The pigment results from metabolic processes that produce compounds interacting with iron in the medium, giving the olive-green to black appearance. This helps set it apart from other gram-positive cocci in anaerobic infections, which typically form nonpigmented colonies. Veillonella parvula, for instance, is gram-negative, not gram-positive, so its colonies wouldn’t match the described staining and pigment pattern. Finegoldia magna and Peptostreptococcus anaerobius are gram-positive cocci as well, but they generally produce nonpigmented colonies rather than the distinctive dark pigment seen with Peptococcus niger.