What is the predominant indigenous flora of the human colon?

What is the predominant indigenous flora of the human colon?

• A. Anaerobic, gram-positive, non-sporeforming bacteria
• B. Aerobic, gram-negative, non-sporeforming bacteria
• C. Aerobic, gram-positive, spore-forming bacteria
• D. Anaerobic, gram-negative, non-spore-forming bacilli

Answer: (D)

The colon is a low-oxygen environment, so its resident bacteria are mainly anaerobes. Among these, the most numerous are Gram-negative, non-spore-forming rods, such as Bacteroides species, which thrive in the nutrient-rich, oxygen-poor conditions of the large intestine. This combination—anaerobic, Gram-negative, non-spore-forming bacilli—best matches the dominant indigenous flora. While other anaerobes exist (including some Gram-positive or spore-forming types), they are not as numerically dominant in the colon as the anaerobic Gram-negative non-spore-forming rods.

The colon is a low-oxygen environment, so its resident bacteria are mainly anaerobes. Among these, the most numerous are Gram-negative, non-spore-forming rods, such as Bacteroides species, which thrive in the nutrient-rich, oxygen-poor conditions of the large intestine. This combination—anaerobic, Gram-negative, non-spore-forming bacilli—best matches the dominant indigenous flora. While other anaerobes exist (including some Gram-positive or spore-forming types), they are not as numerically dominant in the colon as the anaerobic Gram-negative non-spore-forming rods.