what had happen when the following were applied: Dye, Mordant, Decoloriser, Counter stain?

The process you're referring to is known as Gram staining, a common technique in microbiology used to differentiate bacterial species into two large groups: Gram-positive and Gram-negative. Here's what happens at each step:

1. Dye: The primary stain, usually crystal violet, is applied to a heat-fixed smear of bacterial cells. All cells will absorb the dye and appear purple under a microscope.

2. Mordant: Gram's iodine is added, which forms a complex between the crystal violet and the iodine. This complex is too large to be washed out of the cells, so all cells remain purple.

3. Decoloriser: An alcohol or acetone solution is added, which dehydrates the peptidoglycan layer of Gram-positive cells, shrinking and tightening it. The large crystal violet-iodine complexes are not able to penetrate this tightened peptidoglycan layer, and are thus trapped in the cell. The thinner peptidoglycan layer of Gram-negative cells cannot retain the large crystal violet-iodine complexes and the color is easily washed out when decolorized.

4. Counter stain: A secondary stain, usually safranin, is applied. This stain will not be absorbed by Gram-positive cells due to the presence of the crystal violet-iodine complex. However, Gram-negative cells, having lost their primary stain (crystal violet) will take up the counterstain and appear red under a microscope.

So, in summary, after these steps, Gram-positive bacteria will appear purple, and Gram-negative bacteria will appear red.