A Gram stain is a staining method that stains the cell walls of bacteria. It was invented by Hans Christian Gram and is named after him. His actual goal was to make bacteria visible in animal tissue. In the beginning, the method only worked with a few bacteria, but it continuously improved. As already discussed in the topic "cell wall", this staining can be used to distinguish between gram-positive and gram-negative bacteria. The staining works in four steps: First, the bacteria are stained with a color called "crystal violet" and then a "Lugol's solution" is added. These two components form a color complex and all bacteria appear dark purple. After that the bacteria are destained with alcohol, but this only works with the Gram-negatives, because the alcohol does not penetrate the thick cell wall of the Gram-positives. Finally, a counterstaining with saffronine is carried out so that the Gram-negatives are also recognizable in color, making them pink/red.
Explanation for children: A Gram stain is a coloring method that stains the cell walls of bacteria. It was invented by Hans Christian Gram and is name after him. The cells are first colored and then decolored again. Depending on how thick the cell wall of the bacteria is, they finally appear in different colors: The coloring and decolorization works on thin cell walls. But the decolorization does not work on thick cell walls because the discoloring agent does not get through. So the color shows us whether it is a thick or a thin cell wall. Since the coloration was invented by Mr. Gram, bacteria with thick cell walls are called "gram-positive" and bacteria with thin cell walls are called "gram-negative".