Lab 7 - Simple Staining

Question 1

what is the purpose of using staining?

Answer 1

to increase the contrast of a specimen against its background.
most bacterial cells are thin and colorless, so they do not absorb or transmit enough light to form an image with low contrast.

Question 2

what are the three types of stains?

Answer 2

-Simple stain
-Negative stain
-Structural or secondary stain

Question 3

what is the simple stain? what are the types of dyes?

Answer 3

When we use a basic dye that is positively charged to stain cells that are negatively charged.
Types of dyes for simple stains: methylene blue, crystal violet, and safranin.

Question 4

what is negative stain? what is the type of dye used?

Answer 4

When we use an acidic dye that is negatively charged that cannot penetrate the cell, but it stains the surrounding area.
Example of acidic dye used: India ink.

Question 5

what are the structural or secondary stains? what are examples?

Answer 5

When we use dyes to increase the contrast and reveal some aspects of a cell’s structure.
Examples: Gram stain, acid stain, and endospores.

Question 6

what are the steps of staining?

Answer 6

1. Form the smear by spreading a thin layer of cells on a glass slide.
2. Air dry: important to make sure that the smear is fully dry; this preserves the cell’s structure and reduces aerosols.
3. Fixation: attaching or “fixing” the cells to the glass slide. this is done by using agents such as heat or chemicals like methanol. This denatures the cells’ surface proteins and causes them to non-covalently bind to the glass slide.
4. Add the stain by drowning the slide in the stain and letting it sit for 30 seconds before pouring deionized water on it and then drying it with the bibulous papers.

Question 7

what are the advantages and disadvantages of fixation?

Answer 7

Advantages:
-Immobilizes the cells so that they can be studied carefully.
-Denatrues “autolytic” or self-lysis enzymes that digest cellular components post-cell death.

Disadvantages:
-Kills the cells; so we cannot study the processes like transport or motility.
-It may alter the morphology like cell size.

Question 8

why should bacterial smears be allowed to air-dry before heat fixation?

Answer 8

because it could cause harmful aerosols. it also preserves the cell’s structure.