Answer:
Peptidoglycan layer.
Explanation:
The thickness of the peptidoglycan protein layer is the main difference between gram negative, and gram positive bacteria. Gram positive bacteria contains thick peptidoglycan layer which retains the gram staining, and making the bacteria purple under the microscope.
Gram negative bacteria contains thin peptidoglycan layer which do not retain the gram staining, and making the bacteria pink under the microscope.
