If we do not clean our teeth and mouth after eating food, then many harmful micro-organisms called A begin to grow and live on the tooth. These micro-organisms act on the substance B present in the left-over food particles sticking to the tooth to form C. The C eats up the parts D and E of the tooth gradually and ultimately make a hole in the tooth. When this hole reaches part F of the tooth, our tooth becomes painful and we get toothache.
What is A?

Answer

**Step-by-Step Solution:**

1. **Understanding the Problem**: The question asks for the identity of harmful microorganisms that grow on teeth when they are not cleaned after eating.

2. **Identifying the Microorganisms**: The harmful microorganisms that begin to grow on the teeth due to leftover food particles are known as bacteria.

3. **Role of Bacteria**: These bacteria feed on the sugars present in the leftover food. When we do not clean our teeth, these sugars remain on the surface and serve as food for the bacteria.

4. **Formation of Acid**: As bacteria consume the sugars, they produce acids as a byproduct. These acids are harmful and begin to degrade the layers of the tooth.

5. **Consequences of Bacterial Growth**: The acids produced by the bacteria eat away at the enamel, dentine, and eventually reach the pulp cavity of the tooth, leading to tooth decay and pain.

6. **Final Answer**: Therefore, the harmful microorganisms referred to as A in the question are **bacteria**.

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