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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.