What characterizes the cell wall of Gram-positive bacteria?

The correct answer highlights the defining feature of Gram-positive bacteria: they possess a thick layer of peptidoglycan in their cell walls. This thick peptidoglycan layer is crucial as it provides structural strength and rigidity, allowing the bacteria to maintain their shape and resist internal osmotic pressure. Furthermore, the presence of this robust peptidoglycan layer is what causes Gram-positive bacteria to retain the crystal violet stain during the Gram staining procedure, resulting in a purple appearance under a microscope.

This characteristic differentiates them from Gram-negative bacteria, which have only a thin layer of peptidoglycan and an outer membrane containing lipopolysaccharides. The thick peptidoglycan layer in Gram-positive organisms also plays a role in their susceptibility to certain antibiotics, such as penicillin, which target the synthesis of the peptidoglycan.