Which organism is commonly part of vaginal flora and may appear as small alpha-hemolytic colonies on SBA after extended incubation?

• A. Bacteroides fragilis
• B. Lactobacillus
• C. Porphyromonas asaccharolytica
• D. Gardnerella vaginalis

Answer: (B)

The main idea is recognizing the typical vaginal resident organisms and how they appear in culture. Lactobacillus species are a primary part of normal vaginal flora; they help maintain the acidic environment by producing lactic acid (and often hydrogen peroxide), which helps inhibit other microbes. In culture on sheep blood agar, lactobacilli are small, often slow-growing Gram-positive rods. When incubation is extended, they can produce a subtle, alpha-hemolytic appearance—giving a greenish tint around colonies—because their hemolysis is partial and becomes more evident with more time.

This makes lactobacilli the best fit for the description. Bacteroides fragilis and Porphyromonas asaccharolytica are anaerobic bacteria more associated with other sites and contexts and wouldn’t typically present as small alpha-hemolytic colonies on this medium after extended incubation. Gardnerella vaginalis is linked with bacterial vaginosis and has a different culture profile (often Gram-variable rods and clue-cell associations) rather than the classic small alpha-hemolytic colonies described here.