Gram-positive bacteria retain a purple color from the crystal violet stain used in the Gram test. Gram-negative bacteria lose this purple color when treated with a solvent, and appear reddish or pinkish when counterstained with safranin or fuchsin.
The cell wall of gram-positive and gram-negative bacteria consists of peptidoglycan, a structural molecule composed of sugars and amino acids. The cell wall of a gram-negative bacterium has an outer membrane that covers the peptidoglycan layer. However, its peptidoglycan layer is much thinner than a gram-positive bacterium's multilayered wall. For this reason, the cell wall of gram-negative bacteria is more prone to mechanical damage than that of gram-positive bacteria.
Gram-positive bacteria have molecules called teichoic and lipoteichoic acids embedded in their peptidoglycan layer. These molecules play a role in transporting nutrients from the external environment to the bacterial cytoplasm (i.e., the substance that fills the cell). Gram-negative bacteria don't have teichoic and lipoteichoic acids. Instead, they have doughnut-shaped proteins called porins, which allow the passage of nutrients needed by gram-negative bacteria.
Gram-positive bacteria are usually susceptible to penicillins, which interfere with the synthesis of peptidoglycan. Gram-negative bacteria tend to be resistant to this class of antibiotics, because the drug cannot penetrate the outer membrane that protects their cell wall.