What type of bacteria is characterized by a thick peptidoglycan layer in its cell wall?

The type of bacteria characterized by a thick peptidoglycan layer in its cell wall is Gram-positive bacteria. This structural feature is a defining characteristic of Gram-positive species, which possess a substantial amount of peptidoglycan, often several layers thick, localized just beneath the cell membrane. This thick layer not only provides rigidity and strength to the bacterial cell wall but also plays a critical role in the organism's ability to retain the crystal violet stain during Gram staining, leading to the characteristic purple coloration associated with Gram-positive bacteria.

In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane that contains lipopolysaccharides, which makes them stain pink during the Gram staining process. Atypical bacteria may have unique cell wall compositions, often lacking peptidoglycan altogether, such as Mycoplasmas that do not possess a cell wall at all. Thus, the presence of a thick peptidoglycan layer specifically points to Gram-positive bacteria as the correct answer.