Analyze the structural differences between gram-positive and gram-negative bacteria based on peptidoglycan thickness.

Gram-negative bacteria have a thick peptidoglycan layer and lack an outer membrane.

Gram-positive bacteria have a thin peptidoglycan layer and an outer membrane.

Both gram-positive and gram-negative bacteria have equally thick peptidoglycan layers.

Gram-positive bacteria have a thick peptidoglycan layer, while gram-negative bacteria have a thin peptidoglycan layer and an outer membrane.