Which of the following is a characteristic of gram-positive bacteria?

Gram-positive bacteria are characterized by having a thick peptidoglycan layer in their cell wall structure. This thick layer is responsible for retaining the crystal violet stain during the Gram staining procedure, which subsequently leads to their characteristic purple coloration under a microscope. The cellular structure of gram-positive bacteria is relatively simple, with the thick peptidoglycan layer surrounded by a cell membrane, enabling these bacteria to maintain their shape and resist certain environmental challenges.

In contrast, other characteristics mentioned pertain to gram-negative bacteria or are inaccurately describing gram-positive bacteria. For instance, a thin peptidoglycan layer and the pink staining result are characteristic of gram-negative bacteria, which have an outer membrane that gram-positive bacteria lack. Additionally, stating that bacteria lack cell walls entirely does not apply to typical bacterial cell structures, as most bacteria, including gram-positive ones, have cell walls that provide important functions in maintaining cell integrity.