Mycoplasmas are the smallest, free-living bacteria—and the only bacteria that exist without a cell wall. Some mycoplasmas have sterols (a type of lipid-lacking fatty acids) in their plasma membranes that provide the strength a membrane needs to maintain cellular integrity without a wall; since mycoplasmas lack cell walls that provide shape and rigidity, they have no definitive forms. For example, Mycoplasma pneumoniae causes a disease known as primary atypical pneumonia (PAP), a mild form of pneumonia confined to the lower respiratory tract. Because mycoplasmas do not have cell walls, penicillin is ineffective in stopping their growth. Instead, a drug called tetracycline, which inhibits protein synthesis, is recommended as the antibiotic of choice for treatment of PAP.