Egg yolk agar precipitation around colonies 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 phospholipase activity, specifically lecithinase. Egg yolk contains lecithin, a phospholipid, and when a bacterium produces lecithinase, it breaks down lecithin around the colony. This hydrolysis releases products that form an insoluble precipitate, producing a visible ring or cloudy zone surrounding the growth. That precipitation around colonies is the telltale sign of lecithinase production.

Lipase would act on lipids but typically yields different patterns on other lipid-containing media, protease activity is observed as changes in protein-rich substrates, and starch hydrolysis is detected with iodine on starch plates, not on egg yolk agar.