What kind of bacteria doesn't have cell walls?

Mycoplasma is a kind of bacteria that don’t have cell walls. To compensate for the lack of cell walls, mycoplasma builds tough cytoplasmic membranes using sterols formed from cholesterol acquired from the environment. The sterols hold the membrane together with great force, contributing to their durability and their higher resistance to rupturing. Mycoplasma have a parasitic or saprophytic lifestyle, living and growing in the controlled osmotic environment of the host cell. This allows them to live without a cell wall. They are killed rapidly when placed in an environment with very low or very high salt concentrations.