Micro Lab 6 - Simple, Negative And Gram Staining

Front 1

Simple stain

Back 1

Uses a single, basic (positively‐charged) dye to
add color to microbial cells so that they can be
easily seen. As a result, all cells are stained the
same color.

Front 2

Differential stain

Back 2

Uses two basic (positively‐charged) dyes to
distinguish between two different microbial groups
or structures by color reaction. As a result, these groups are stained in two contrasting colors.

Front 3

Negative stain

Back 3

Uses a single, acidic (negatively‐charged) dye to
color the background, leaving the microbe unstained. As a result, cells appear colorless against a stained background.

Front 4

Gram stain

Back 4

A differential stain for bacteria useful in identification. Gram‐positive organisms appear purple from the crystal violet‐mordant retention, whereas Gram‐negative organisms appear red due to the loss of crystal violet and counterstaining with safranin.

Front 5

Cell size, shape, and arrangement.

Back 5

What information can be obtained from simple staining?

Front 6

The *heat coagulates the microbes’ proteins,
causing the bacteria to stick to the slide.

Back 6

What is the purpose of heat fixation?

Front 7

Negatively‐charged acidic dyes are repelled by the negatively‐charged bacterial surface, forming
a deposit around the organism. As a result, the *background is *stained, and the *microbe is
*transparent

Back 7

Why doesn’t the negative stain (*nigrosin) colorize the cells?

Front 8

To determine the *morphology & *arrangement of bacteria too delicate to withstand *heat‐fixing.

Back 8

What is the purpose of *negative staining?

Front 9

Iodine forms an intracellular complex with crystal
violet, making the dye molecules *larger and
less likely to leave the *Gram‐positive cell during subsequent washings. As a result, the *mordant
enhances the retention of *crystal violet in Gram‐positive bacteria.

Back 9

During the Gram stain, what is the purpose of the mordant (*iodine)?

Front 10

During decolorization, *Gram‐negative bacteria *lose the crystal violet stain, while *Gram‐positive cells are *NOT decolorized. This is what makes the Gram stain differential. However, overdecolorization
can lead to misleading results, since overexposure
to alcohol can also *strip crystal violet from Gram‐positive cells, making them appear Gram‐negative.

Back 10

During the Gram stain, why is decolorization (*95% alcohol) the most critical step?

Front 11

Simple, Negative and Gram Staining

Back 11

Micro Lab 6 - is