Differential Stain Microbiology Lab Report

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Midterm Lab Practicum
Name:
MICRO20
East Los Angeles College
Date:

Running head: Midterm Lab Practicum Report

Midterm Lab Practicum
Gram Stain
The gram stain is a differential stain that utilizes a decolorization step between two
basic stains. Crystal violet is the primary stain. This differential stain is used to characterize
bacteria into either gram-positive bacteria or gram-negative bacteria.
Crystal violet stain both bram negative organisms and gram positives when iodine is
used as a mordant. However, decolorization with acetone or alcohol detaches the crystal
violet from the gram-negative cells, while the gram-positive cells are not decolorized.
The presence of a thick peptidoglycan layer in the inner cell wall of gram positive
bacteria makes them stain violet. The gram-negative bacteria stain pink/red because of their
thinner peptidoglycan layer in the cell wall.

Method.
1. Start with up to 3 heat-fixed emulsions. Place the slide on a rack over a staining tray.
2. Cover the smears (not the whole slide) with crystal-violet stain. Ensure the staining
tray catches all excess stain.
3. Use the slide holder to hold the slide at an angle, then gently rinse the slide with
distilled water.
4. Cover the smears with gram iodine stain for a minute. Ensure the staining tray catches
all the excess stain.
5. Use the slide holder to hold the slide at an angle. Rinse the slide using distilled water.
6. Holding the slide at an angle, decolorize with gram decolorizer, allowing it to trickle
down the slide until the runoff is clear.
7. When the runoff is clear, rinse the slide with diluted water into the staining tray
immediately.
8. Counterstain the smears with safranin for 1 minute.
9. Grasp the slide with a slide holder and hold it at an angle. Use distilled water to rinse
the slide.
10. Blot dry gently in a tablet of bibulous paper or paper towels. Don’t rub. When dry,
observe under oil immersion.

1

Running head: Midterm Lab Practicum Report

Data

Shape
Arrangement
Gram stain reaction
Bacilli shaped
streptobacilli
Gram-positive reaction
Analysis: the slide was composed of gram-positive streptobacilli
Conclusion: gram staining is important to determine the type of bacteria and their shape and
arrangement. The result of our staining procedure was gram-positive streptobacilli.
Acid Fast Stain
Purpose
It is a distinctive stain used to trace the presence of cells capable of retaining a primary
stain when treated with acid alcohol. Examples of clinically important acid-fast bacteria
include Mycobacterium tuberculosis and Mycobacterium leprae which cause tuberculosis and
leprosy ...


Anonymous
Excellent resource! Really helped me get the gist of things.

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