Scientists divide bacteria into so-called "gram positive" varieties like Strep, which lack rigid cell walls and can be stained with dye, and "gram negative" ones that have cell walls and aren't dyeable.

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They tested their treated materials with Staphylococcus aureus and Escherichia coli, the gram-positive and gram-negative bacteria that are always giving humans a bad day.

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There is particular concern about Gram-negative bacteria (like E.coli and K. pneumonia), due to their outer membrane which can prevent some antibiotics from penetrating the cell.

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