Which bacterial groups generate secondary bile acids and are they Gram-positive or Gram-negative?

• A. Bacteroides fragilis and Fusobacterium; Gram-negative
• B. Clostridium clusters XIVa and VIII and Eubacterium; Gram-positive
• C. Lactobacillus and Streptococcus; Gram-positive
• D. Escherichia and Enterococcus; Gram-negative

Answer: (B)

Secondary bile acids are produced in the gut by specific anaerobic bacteria that carry out 7α-dehydroxylation of primary bile acids. The main producers are Gram-positive anaerobes, particularly Clostridium clusters XIVa and VIII and Eubacterium. These organisms harbor the enzymes needed to convert primary bile acids like cholic and chenodeoxycholic acid into secondary forms such as deoxycholic and lithocholic acid.

This is why that grouping is the best fit: it identifies the Gram-positive, anaerobic bacteria known to drive the formation of secondary bile acids. Other groups listed either are not the primary 7α-dehydroxylators or include Gram-negative bacteria, which do not match the key producers of secondary bile acids.