L15. GRAM STAINING

L15. Gram staining

1. Introduction

Gram staining is a method of differentiating bacterial spieces into two large groups, gram positive and gram negarive. This differentation is based by the chemical and physical properties the their cell walls by detecting a peptidoglycan, which is present in a thick layer in gram -  positive bacteria.

Gram - negative: color pink
Gram - positive: color purple

2. Material

• 1 slide
• 1 cover slip
• Tongs
• Needle
• Gram stain
• Decolorize reagent : 96% ethanol
• Microscope
• Yogurt

3. Objectives

• Differentiate yogurt bacteria.
• Relate the staining procedure with the structure of the cells.

4. Procedure

First of all, prepare a heat-fixed sample of the bacteria to be stained and cover the smear with crystal violet for an exposure of 1 min.

After, rinse with distilled water and apply iodine solution for 1 min and again, rinse the sample with distilled water.

Decolorize using ethanol. Drop by drop until the purpule stops flowing. Wash immediately with disitilled water.

Cover the sample  with the safranin stain for an exposure time of 45 seconds and rinse the shample with distilled water.

Finally, gently dry the slide with paper.