Small alpha-hemolytic colonies on a sheep blood agar plate after 48 hours of incubation from a vaginal culture describe which organism?

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

Answer: (C)

A key idea here is the normal vaginal microbiota and how it appears in culture. Lactobacillus species dominate the healthy vaginal flora; they are Gram-positive rods that grow well on blood agar and often form small colonies. With extended incubation, some Lactobacillus strains can show a light alpha-hemolytic zone around colonies on sheep blood agar. This pattern fits a vaginal culture from a healthy baseline.

The other organisms listed are not typical vaginal commensals and have different growth requirements or appearances: Bacteroides and Porphyromonas are anaerobic Gram-negative rods that tend to require strict anaerobic conditions and show colony types not described as small alpha-hemolytic colonies on standard sheep blood plates; Clostridium is an anaerobic, spore-forming genus with distinct colony and toxin characteristics that don’t match this presentation.

So, the small alpha-hemolytic colonies observed after 48 hours on sheep blood agar from a vaginal culture best describe Lactobacillus.