What does the casein (skim milk) hydrolysis test show about microbial proteolysis?

What does the casein (skim milk) hydrolysis test show about microbial proteolysis?

• A. Clearing zones around colonies indicate casein proteolysis by proteases.
• B. Clearing on skim milk indicates lipase activity.
• C. Color change indicates lactase production.
• D. No clearing indicates proteolysis.

Answer: (A)

Proteolysis of casein by microbes is shown by clearing around the colony on skim milk agar. Skim milk contains casein, which makes the medium opaque. When a microbe secretes proteases that digest casein, the surrounding medium loses that opacity and a clear zone appears where the casein has been hydrolyzed. This clearing directly indicates extracellular proteolytic activity against casein. If no protease is produced, the medium stays opaque with no clearing. Activities like lipase or lactase would require different substrates and would not produce the same clear zones in this test.

Proteolysis of casein by microbes is shown by clearing around the colony on skim milk agar. Skim milk contains casein, which makes the medium opaque. When a microbe secretes proteases that digest casein, the surrounding medium loses that opacity and a clear zone appears where the casein has been hydrolyzed. This clearing directly indicates extracellular proteolytic activity against casein. If no protease is produced, the medium stays opaque with no clearing. Activities like lipase or lactase would require different substrates and would not produce the same clear zones in this test.