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Stains purple in a Gram stain because it lacks the extra peptidoglycan layer in the membrane, so the crystal violet used in a Gram stain is able to penetrate the cell.

They will appear purple under a microscope. Less harmful bacteria tend to be Gram positive. It's easy to kill because it has thinner membrane.

Actually, both gram positive and gram negative bacteria have just one peptidoglycan layer. The gram positive bacteria have the peptidoglycan layer on the outside, and that is what stains when you do a Gram stain. The crystal violet doesn't go past the peptidoglycan layer. They're easier to kill in most cases because they don't have the extra peptidoglycan layer, and since we don't have any peptidoglycan in our bodies, our immune system can immediately recognize them as foreign. Plus they're more susceptible to penicillin.

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