Which of the following bacteria is characterized by a thicker peptidoglycan layer?

The presence of a thicker peptidoglycan layer is a defining characteristic of Gram-positive bacteria. In Gram-positive bacteria, the cell wall is composed of a substantial layer of peptidoglycan, which can be up to 90% of the cell wall structure. This thick peptidoglycan layer retains the crystal violet stain used in the Gram staining process, resulting in a purple appearance under the microscope.

In contrast, Gram-negative bacteria have a much thinner peptidoglycan layer, typically around 10%, and they possess an outer membrane that contains lipopolysaccharides. This structural composition accounts for their inability to retain the crystal violet stain, which leads to a pink coloration after counterstaining.

Mycoplasma, on the other hand, is unique in that it lacks a cell wall altogether, which means it does not have peptidoglycan. This absence renders the standard Gram staining ineffective for identifying these organisms, as they will not fall into either category of Gram-positive or Gram-negative based on cell wall composition.

Therefore, the option indicating Gram-positive bacteria is correct because their defining feature is indeed the presence of a thicker peptidoglycan layer, distinguishing them from other bacterial types.