Bacteria are classified as Gram-positive or Gram-negative based on:

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Bacteria are classified as Gram-positive or Gram-negative based on the staining characteristics of their cell wall. Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which retains the crystal violet stain, and appear purple in color under a microscope. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, which does not retain the crystal violet stain but instead takes up the counterstain safranin, and appear pink or red in color under a microscope.

Bacteria Are Classified as Gram-Positive or Gram-Negative Based On