Final answer:
Mycoplasmas are unique among bacteria because they do not have a cell wall, which distinguishes them from both gram-positive and gram-negative bacteria.
Step-by-step explanation:
Mycoplasmas differ from other bacteria in that they lack a cell wall. These tiny bacteria are among the smallest known and can be free-living or pathogenic. Unlike gram-positive bacteria that have a thick peptidoglycan layer, or gram-negative bacteria with a thin peptidoglycan layer and an outer membrane, mycoplasmas do not have these structural components because they are devoid of a cell wall entirely.