Which is a characteristic of Gram-positive bacteria?

Gram-positive bacteria are characterized by their thick peptidoglycan layer, which is a major structural component of their cell wall. One of the defining features of Gram-positive bacteria is their ability to retain the crystal violet stain used in the Gram staining procedure. When stained, the thick peptidoglycan layer traps the crystal violet, causing the bacteria to appear purple under a microscope. This property is crucial for the classification of bacteria and differentiates Gram-positive organisms from Gram-negative bacteria, which have a thinner peptidoglycan layer and an outer membrane that prevents the retention of the crystal violet stain.

The other characteristics mentioned do not apply to Gram-positive bacteria: they do not have a thin peptidoglycan layer, nor do they possess an outer membrane, which is typical of Gram-negative bacteria. Additionally, the assertion that Gram-positive bacteria are always pathogenic is not accurate, as many Gram-positive species are non-pathogenic and some even play beneficial roles in the environment and human microbiota. This makes the ability to retain the crystal violet stain a definitive characteristic of Gram-positive bacteria.