Mycoplasmas are neither gram-positive nor gram-negative bacteria. Although they do not have a cell wall of peptidoglycan, mycoplasmas have a sterol-packed (sterols are like cholesterol) cell membrane that protects them from the environment.
Because mycoplasmas lack cell walls, antibiotics like penicillin and cephalosporin that target the biosynthesis of bacterial murein are ineffective. The antibiotics tetracycline and erythromycin are also powerless in that they aim to hurt bacterial ribosomes that make the peptidoglycan components.
Mycoplasmas can be described as pleomorphic because they do not have a rigid shape. They can stretch and change their shape because they lack murein or peptidoglycan in their cell walls. In ...