Step 1. Prepare a slide with the culture by transferring the specimen to be examined onto a drop of suspension medium (distilled water) using an inoculation loop. Spread the specimen on the slide to ensure that it is not clumped.
Step 2. Fix the culture by heating the slide over a Bunsen burner to evaporate the water -- make sure not to hold the slide over the flame too long or it will denature the specimen.
Step 3. Drop a few drops of crystal violet stain onto the fixed culture (enough to cover the specimen) and let it stand for 20 seconds. Then pour off the crystal violet stain and gently rinse the excess stain with distilled water.
The objective of this step is:
To allow the crystal violet stain to bind to the peptidoglycan molecules of the Gram + bacteria (if present). Remember, Gram + bacteria have a large peptidoglycan layer located outside the bacterial "inner membrane".
To allow the crystal violet stain to bind to the lipopolysaccharide molecules attached to the "outer membrane" of the Gram - bacteria (if present). Remember, while Gram + bacteria have no "outer membrane", Gram - bacteria have lipopolysaccharide molecules attached to their bacterial "outer membrane".
Step 4. Drop a few drops of iodine solution on the smear and let it stand for 20 seconds. Then pour off the iodine solution and rinse the slide with distilled water.
The objective of this step is to fix the crystal violet to the peptidoglycan molecules on the Gram + bacteria.
Step 5. Drop a few drops of decolorizer (acetone or ethanol) and let the solution trickle down off the slide until the decolorizer has removed enough of the color to drip off clear. Then IMMEDIATELY rinse the slide off with distilled water after 5 seconds. Note that pouring too much decolorier will cause the decolorization of the Gram + bacterial cells (in addition to the Gram - bacteria), and the purpose of staining will be defeated.
The objective of this step is to dissolve the lipopolysaccharide membraine in the Gram - bacteria and expose the thin peptidoglycan layer below.
Step 6. Drop a few drops of basic counterstain (fuchsin or safranin) on the slide and let it sit for 20 seconds, then wash off the solution with distilled water.
The objective of this step is to stain the peptidoglycan layer of the Gram - bacteria a pink / red color. Remember that the addition of iodine to the crystal violet in Step 4 binds the crystal violet stain in the Gram + bacteria, so the counterstain is unable to bind to the peptidoglycan wall in the Gram + bacteria in the specimen.
Step 7. Observe the slide under light microscopy.