Egg yolk agar showing a precipitate around the colony indicates production of which enzyme?

• A. Lecithinase production
• B. Lipase production
• C. Protease activity
• D. Starch hydrolysis

Answer: (A)

Egg yolk agar is used to detect lecithinase (phospholipase C) activity. If a bacterium produces lecithinase, it hydrolyzes lecithin in the egg yolk, and the breakdown products form an opaque, precipitate-like zone around the colony. That precipitate around the colony is the hallmark of lecithinase production. Lipase would alter lipid substrates in different contexts and isn’t identified by this specific precipitate pattern on egg yolk; starch hydrolysis is shown by clearing after iodine treatment on a starch-containing plate, and protease activity is observed as clearing in protein-containing media.