Which Gram type has a thick peptidoglycan layer and no outer membrane?

• A. Gram-negative
• B. Gram-variable
• C. Gram-negative with no outer membrane
• D. Gram-positive

Answer: (D)

The main idea is how bacterial cell wall structure determines Gram staining. Bacteria with a thick peptidoglycan layer and no outer membrane retain the dye during the stain process, so they appear purple under a Gram stain. The dense, thick wall traps the crystal violet–iodine complex and isn’t easily washed out by alcohol, so no outer membrane is needed to keep the stain.

In contrast, bacteria with an outer membrane and only a thin peptidoglycan layer don’t hold onto the dye as well after alcohol treatment; the outer membrane is disrupted and the dye is washed away, leaving the pink counterstain. Gram-variable bacteria don’t consistently fit this pattern because their cell-wall properties can vary, leading to inconsistent staining.

So, the description fits Gram-positive: thick peptidoglycan with no outer membrane.