They are everywhere. Total weight of all bacteria in the world estimated to outweigh all other organisms combined. Any mutation can become established really quickly. Can also get variation in population through recombination. No membrane-bound organelles .
Walls can vary.
Gram negative bacterial peptidoglycan wall not as thick, but have extra layer of lipopolysaccharides in wall. Glycocalyx outside of wall layer of polysaccharides. Only made in certain environments. Capsule - outside of wall, gelatinous layer but not bound so tight. Protect the bacterium, give stability in shifting environment.