The selective nature of Hektoen enteric agar is due to the inclusion of which component?

• A. Bile salts
• B. Sodium metabisulfite
• C. Bromothymol blue
• D. Sodium chloride

Answer: (A)

Bile salts provide the selective pressure in Hektoen enteric agar. They inhibit most Gram-positive bacteria and many non-enteric organisms, while Gram-negative enteric bacilli can grow, so the medium favors the growth of the target group. This selective environment is what lets you isolate enteric pathogens and then differentiate them later based on fermentation patterns and hydrogen sulfide production. The other components mainly contribute to differentiation and indicators (for example, pH indicators show lactose/sugar fermentation, and ferric ammonium citrate detects H2S), or simply support growth, but they don’t establish the selectivity.