What distinguishes Gram-positive bacteria from Gram-negative bacteria?

Gram-positive bacteria are primarily distinguished from Gram-negative bacteria based on three key characteristics: the thickness of their peptidoglycan layer, the presence of an outer membrane, and their ability to retain the crystal violet stain during the Gram staining process.

The thick peptidoglycan layer found in Gram-positive bacteria provides structural support and protection. This thick wall retains the crystal violet dye used in the Gram staining method, which leads to a purple coloration under the microscope. In contrast, Gram-negative bacteria have a much thinner peptidoglycan layer, which does not retain the crystal violet stain after the additional step of adding a decolorizer.

Additionally, Gram-negative bacteria possess an outer membrane that Gram-positive bacteria do not have. This outer membrane contains lipopolysaccharides, which can contribute to their pathogenicity and can result in different staining characteristics.

Therefore, all the distinctions—presence of an outer membrane, the thickness of the peptidoglycan layer, and the ability to retain the crystal violet stain—collectively define the key differences between these two groups of bacteria. This makes "All of the above" the most accurate response, as it encapsulates all defining features of Gram-positive bacteria versus Gram-negative bacteria.