Gram staining E.coli bacteria

All bacteria can be divided into two groups according to their surface structure, gram positive and gram negative.

source: Microbeonline

Gram positive bacteria stain purple because of the thick layer of peptidoglycan in their cell wall, gram negative bacteria stain pink because their layer of peptidoglycan is thinner.

Preparing sample for staining:

1. Pass a wire loop through a Bunsen flame and let it cool. Transfer a loopful of milk to a clean dry slide and spread the milk thinly and evenly over the slide. Allow the slide to dry.

2. Hold the slide on one side and pass it through the Bunsen flame (4 times) to fix the bacteria. Do not overheat the slide.

Staining process:

1. Place the slide over a small beaker and stain with crystal violet for 30 s.

2. Wash off the stain gently with distilled water and flood the smear again with the mordant iodine solution for 60 s.

3. Wash off the stain and mordant gently with distilled water followed by several drops of decolourizer (alcohol). Flood the smear again with the counterstain safranin for 15 s.

4. Wash off the counterstain gently with distilled water and allow the slide to dry. Place a drop of immersion oil on the dry smear. Place the slide on the stage of the microscope and use the oil immersion objective to locate and observe the cells.

Conclusion: E.coli is a gram negative bacteria.