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It loses its violet color after a Gram stain because the violet does not get into the cells.

It possesses Gram negative cell wall, which has an extra layer of peptidoglycan surrounded by a lipopolysaccharide outer membrane. The Gram negative bacteria tends to be harder to kill because of that extra layer, and during staining, the purple coloring does not get through the membrane, leaving only Gram positive bacteria purple under the view of the microscope.

The gram negative bacteria don't have an extra peptidoglycan layer, they have one layer, the same as gram positive bacteria. The difference is that gram negative bacteria have an extra outer membrane, outside of the peptidoglycan layer. The Gram stain can't enter the peptidoglycan layer, because of the outer membrane. They can be more dangerous than gram positive bacteria because sometimes they have a capsule or layer of slime that camouflages the bacteria so our immune system doesn't recognize them. Gram negative bacteria are usually immune to penicillin, too.

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