Gram stain differentiates bacteria into gram positive and gram negative bacteria. The primary stain here is crystal violet which stains the peptidoglycan cell wall bluish-purple in color. Since both types of bacteria have peptidoglycan in their cell wall, they both get stained by crystal violet.
It is however the next steps which result in their differentiation. Gram positive bacteria has a thicker layer of peptidoglycan than gram negative bacteria. So when washed with ethanol, it retains its blue color whereas gram negative bacteria decolorizes. Hence, in the next step the counter stain (mostly safranin) is taken up by gram negative bacteria and it shows red color whereas gram positive bacteria still remains blue.
B) stain only the thicker peptidoglycan cell walls
Explanation:
Gram staining is a technique which was developed by the Hans Christian Gram in 1884 to distinguish the bacterial strains. Based on the colour absorbed and retained by the bacteria, the bacteria are distinguished either as gram-positive or gram-negative.
He used a colour stain- crystal violet which is a positive dye and binds to the cell wall made of peptidoglycan. The crystal violet binds to the cell wall with the help of a mordant called iodine. If this dye retains inside and the cell appears purple then the cell is considered gram-positive else negative.
