What does a clear zone around colonies on an egg yolk agar suggest about Enterobacteriaceae?

What does a clear zone around colonies on an egg yolk agar suggest about Enterobacteriaceae?

• A. It indicates ammonia production
• B. It indicates lecithinase production
• C. It indicates carbohydrate fermentation
• D. It indicates protein hydrolysis

Answer: (B)

A clear zone around colonies on egg yolk agar suggests lecithinase production by the bacteria present. Lecithinase, also known as phospholipase, is an enzyme that breaks down lecithin, a phospholipid component of the yolk. As the bacteria grow and secrete lecithinase, the lecithin in the egg yolk is hydrolyzed, leading to a clearing effect around the colonies. This is a characteristic indicator used to identify certain bacteria within the Enterobacteriaceae family. The clear zone specifically signifies the breakdown of lecithin rather than ammonia production, carbohydrate fermentation, or protein hydrolysis. Ammonia production typically results in alkaline changes that would not specifically produce a clear area, while carbohydrate fermentation usually leads to color changes in the medium rather than clearing. Protein hydrolysis, on the other hand, would involve the breakdown of proteins, which would not be demonstrated by a clear zone on egg yolk agar. Thus, the presence of a clear zone is indicative of lecithinase activity and is a vital consideration in the identification of specific Enterobacteriaceae members.

A clear zone around colonies on egg yolk agar suggests lecithinase production by the bacteria present. Lecithinase, also known as phospholipase, is an enzyme that breaks down lecithin, a phospholipid component of the yolk. As the bacteria grow and secrete lecithinase, the lecithin in the egg yolk is hydrolyzed, leading to a clearing effect around the colonies. This is a characteristic indicator used to identify certain bacteria within the Enterobacteriaceae family.

The clear zone specifically signifies the breakdown of lecithin rather than ammonia production, carbohydrate fermentation, or protein hydrolysis. Ammonia production typically results in alkaline changes that would not specifically produce a clear area, while carbohydrate fermentation usually leads to color changes in the medium rather than clearing. Protein hydrolysis, on the other hand, would involve the breakdown of proteins, which would not be demonstrated by a clear zone on egg yolk agar. Thus, the presence of a clear zone is indicative of lecithinase activity and is a vital consideration in the identification of specific Enterobacteriaceae members.