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Microbiology · Chapter 2

The Gram Stain

This is a working bench, not a slideshow. The smear holds a thick-walled cell and a thin-walled cell. Add the four reagents in any order you like and set the decolorizer timer yourself. The slide keeps track of how much dye each cell still holds, so a skipped step or a long alcohol rinse gives you the wrong answer, exactly as it would at the bench.

The smearnothing added yet

A microscope field holding Gram-positive cocci and Gram-negative rods during the stain
peptidoglycan wall outer membrane crystal violet safranin

Inside the walldye trapped in the mesh

Cross sections of a thick Gram-positive wall and a thin Gram-negative wall with its outer membrane

Nothing on the slide yetpick a reagent to start

Reagentsany order you like

Decolorizer timer5 seconds
1 s5 s12 s25 s

Read the slide

Thick wallno colortruly Gram-positive
Thin wall and outer membraneno colortruly Gram-negative
Add the reagents, then read what the slide reports.
What you did