Small alpha-hemolytic colonies are seen on a sheep blood agar plate after 48 hours of incubation on a vaginal culture. This describes which organism?

• A. Porphyromonas
• B. Lactobacillus
• C. Clostridium
• D. Bacteroides

Answer: (B)

The key idea is recognizing the typical vaginal normal flora. Lactobacillus species dominate the healthy vaginal microbiome and are Gram-positive rods that tolerate the acidic environment they help create by producing lactic acid. On sheep blood agar, they often form small colonies, and after extended incubation (around 48 hours) can show a subtle alpha-hemolytic appearance—greenish coloration around colonies due to partial hemolysis. In a vaginal culture, this description—small colonies with alpha-hemolysis after 48 hours—is classic for Lactobacillus.

The other organisms listed aren’t the usual culprits for this specific growth pattern in a vaginal sample. Porphyromonas and Bacteroides are anaerobic, often associated with oral or gut/intra-abdominal sites, and wouldn’t be expected as small alpha-hemolytic colonies on a routine vaginal culture. Clostridium is also an anaerobe with different colony characteristics and clinical contexts.

So, given the site (vagina) and the growth pattern (small alpha-hemolytic colonies after 48 hours on sheep blood agar), Lactobacillus best fits the description. It reflects the normal, protective vaginal flora rather than a pathogenic anaerobe.