Final answer:
The difference in Gram stain between Gram-positive and Gram-negative bacteria is due to the presence and thickness of the peptidoglycan layer in their cell walls; Gram-positive bacteria have a thick peptidoglycan layer while Gram-negative bacteria have a thin one underneath an outer membrane.
Step-by-step explanation:
Bacteria classification through Gram staining reveals differences in the cell wall structure, which is responsible for the varying reactions to the stain. Gram-positive bacteria have a thick cell wall consisting largely of peptidoglycan, often containing teichoic acids which are sometimes linked to the plasma membrane as lipoteichoic acids, anchoring the cell wall to the membrane. In contrast, Gram-negative bacteria possess a thinner layer of peptidoglycan, beneath an outer membrane that includes lipopolysaccharides (LPS) and lipoproteins. This outer layer is distinct from the cell wall and contributes to the characteristic Gram-negative staining.
In response to the question regarding the reason for the difference in the Gram stain between Gram-positive and Gram-negative bacteria, the correct answer is: a) Presence of peptidoglycan layer.