The Gram stain is the first thing a lab does to a new isolate, and its answer — purple or pink — can steer antibiotics within minutes. Four reagents are added in a fixed order. Step through them and watch a thick-walled and a thin-walled cell part ways.
The smear
peptidoglycan wallouter membrane
The four stepspress Next to begin
Gram-positive— thick wall
Gram-negative— thin wall + OM
Order matters: “Come In And Stain” — Crystal violet · Iodine · Alcohol · Safranin.