Bacterial smears are fixed before staining to make their walls permeable.
accept stain.
affix the cells to the slide.
make the cells visible.
The Correct Answer and Explanation is:
Correct Answer: affix the cells to the slide.
Explanation:
Fixation is a crucial step in the preparation of bacterial smears before staining. The primary purpose of fixing a bacterial smear is to affix, or attach, the bacterial cells to the microscope slide, ensuring they remain in place during the staining and rinsing procedures.
When a smear is prepared, a small amount of bacteria is spread on the surface of a glass slide, often mixed with a drop of water to thin the sample. Once the smear is allowed to air dry, it is “fixed” by either passing the slide through a flame (heat fixation) or by using a chemical fixative, such as methanol. This process causes the cells to adhere to the slide.
Here’s why this is necessary:
- During the staining process, dyes such as crystal violet or safranin are applied to the slide, often followed by rinsing with water or other solvents.
- If the bacteria were not fixed to the slide, they could be washed away during these steps, resulting in an unsuccessful stain and an inability to observe them under the microscope.
- Heat fixation also kills the bacteria, which not only makes handling the slide safer but also helps to preserve the overall structure of the cells.
While fixation does slightly alter the cell’s structure, it generally maintains the basic shape and arrangement of the bacterial cells, allowing microbiologists to observe important characteristics such as shape (coccus, bacillus, spirillum) and arrangement (chains, clusters, pairs).
Fixation is not intended to make the cell walls permeable, accept stain, or make the cells visible per se. These functions are primarily achieved by the staining process itself. The stain provides contrast by coloring the cells (or parts of them), making them visible under the microscope.
In summary, the fixation step ensures that bacterial cells remain affixed to the slide during staining and washing procedures, enabling accurate microscopic examination.