Gram Staining

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Micro Biology Lab
Gram Staining Lab Report


The purpose of this lab was to experiment with a staining technique called Gram Staining. Gram Stains is used to identify and classify bacteria, it is a differential stain that allows you to classify bacteria as either gram-positive or gram-negative. The foloowing is a breif summerary of the gram staining procedure. Apply the primary stain

Apply Mordant
Apply decolorizing agent
Apply the secondary stain

Bacteria that lose color easily after decolorization are referred to as gram-negative, and bacteria that retain the color from the primary stain is called gram-positive. Bacteria stain differently because of chemical and physical differences in the cell wall.


For this lab we used three cultures; Escherichia Coli, Bacillus Subtilis, and Staphylococcus Epidermidis. Using a different slide for each sample we started by preparing and fixing each slide. Next we covered the slide with crystal violet for 30 seconds then rinsed with water. Next we covered the slide with Grams Iodine for 10 seconds and rinsed with water. The next step is to decolorize the slide with ethanol, using a small amount on the slide for no more than ten seconds, and rinsing with water. The last stain we needed to put on the slide was Safranin for 30 seconds,and rinsed with water again. After blotting the slide dry we began to examine the slides under a microscope. We first began at 10x magnification to find the general area were the sample was on the slide. Then we moved to 100x magnification and used oil immersion.


When viewing the slide of E. Coli I could see that the stain was pinkish red and the general shape of the bacteria were rod-shaped or Bacillus. The pinkish red color of the slide indicates that the sample was Gram-negative. For the slide containing Staphylococcus epidermidis the stain appeared to be purple in color, indicating the bacteria was gram positive. The general...
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