Ivy Tech Microbiology Lab Test 2 Practice

Which statement about casein hydrolysis is most directly indicated by clearing around colonies on skim milk agar?

Lipase activity on fats. Proteolysis of casein indicated by clearing around colonies. Growth inhibition due to casein. Clearing around colonies indicates proteolysis of starch.

Correct Answer:

Clearing around colonies indicates proteolysis of starch.

Explanation:

On skim milk agar, casein gives the medium its opaque, milky look. If a microbe produces proteases that hydrolyze casein, the surrounding area becomes clear because the casein is broken down and can no longer scatter light. That clear halo directly indicates proteolysis of casein. This is not a sign of lipase activity on fats, nor does clearing imply growth inhibition. Clearing due to starch hydrolysis would be seen on starch-containing media with a starch-iodine test, not on skim milk agar.

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