* Bacteria don't grow "heavily" on starch agar because they produce alpha amylase. Instead, the opposite is true. Bacteria produce alpha amylase because they grow on starch agar.

* Starch agar is a medium that contains starch as the primary carbon source. Bacteria that can utilize starch will grow on this agar.

* Alpha amylase is an enzyme that breaks down starch into simpler sugars. Bacteria that produce alpha amylase can use these sugars as food.

Here's a more detailed explanation:

1. Starch agar: This medium is made with starch as the primary energy source. It contains other nutrients needed for bacterial growth but the starch is the key component.

2. Bacteria and starch utilization: Some bacteria have the ability to break down starch. They do this by producing an enzyme called alpha amylase.

3. Alpha amylase breakdown: Alpha amylase hydrolyzes (breaks down) the starch molecules into smaller sugars like glucose and maltose.

4. Bacterial growth: The bacteria can then use these sugars as energy and building blocks for their growth.

So, the reason bacteria grow on starch agar is that they can use the starch as a food source, and they produce alpha amylase to help them break down the starch.

The presence of alpha amylase is an indication that the bacteria are able to utilize starch and grow on starch agar.

Let me know if you have any other questions.