What distinguishes a Gram-positive bacterium from a Gram-negative bacterium?

The distinguishing feature between Gram-positive and Gram-negative bacteria is indeed the presence of a thick peptidoglycan layer in the cell wall of Gram-positive bacteria. This thick layer retains the crystal violet stain used during the Gram staining procedure, which gives these bacteria their characteristic deep purple color.

In contrast, Gram-negative bacteria have a much thinner layer of peptidoglycan and an outer membrane that contains lipopolysaccharides. This structure enables them to appear pink after Gram staining due to the counterstaining with safranin, as the initial crystal violet stain is washed out. The thick peptidoglycan layer not only plays a crucial role in maintaining cell shape and structural integrity but also acts as a protective barrier against certain antibiotics and detergents.

While other features like the ability to form spores and differences in cell size may be relevant for defining specific bacterial characteristics, they do not serve as distinguishing factors in the Gram staining process between Gram-positive and Gram-negative bacteria.