When treated with crystal violet and subsequently with iodine some bacteria remain colourless and these are called Gram ........ bacteria.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Gram Staining Process**: The question refers to a specific procedure in microbiology known as Gram staining, which is used to classify bacteria into two groups based on their cell wall structure.

2. **Crystal Violet Treatment**: In the Gram staining process, bacteria are first treated with a dye called crystal violet. This dye is absorbed by the bacterial cells.

3. **Iodine Treatment**: After the crystal violet treatment, iodine is applied. Iodine acts as a mordant, which helps to fix the dye inside the cells.

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