1. gram-positive bacteria are colored purple when the gram stain binds to which composes their thick cell wall.

Gram staining is used to differentiate bacteria by the physical and chemical properties of its cell wall by detection of peptidoglycan. The cell wall of Gram-positive bacteria have a higher peptidoglycan and lower lipid content than Gram-negative bacteria which allows the former to retain its purple apperance after the primary stain of crystal violet. Bacteria is stained by crystal violet that sticks to their cell wall giving it colored purple appearance.