Edward and Freundt present a comprehensive taxonomic classification and nomenclature system for pleuropneumonia-like organisms (PPLO), addressing decades of confusion in the field. The authors examine the organisms' relationships to bacteria, viruses, and L-phase bacterial variants, concluding that PPLO differ sufficiently from these groups to warrant separate classification. Key distinguishing features include lack of a rigid cell wall (bounded only by delicate membrane), absolute penicillin resistance independent of penicillinase, cholesterol requirement for growth, and inhibition by antiserum without complement involvement. Morphologically, organisms exhibit mycelial growth with reproduction through elementary bodies, differing from bacterial binary fission and L-phase multiplication patterns. The authors propose establishing the order Mollicutales (or alternatively Mycoplasmatales), family Mycoplasmataceae, and genus Mycoplasma with Mycoplasma mycoides as the type species. Fifteen established species are described, isolated from cattle, goats, dogs, rats, mice, chickens, humans, and swine. The classification consolidates parasitic and saprophytic strains within a single genus, rejecting earlier systems based on host specificity. This unified nomenclature aims to facilitate future research and provide standard terminology for the growing number of newly isolated PPLO strains.

Key findings

• Pleuropneumonia-like organisms lack rigid cell walls and are bounded by delicate membranes, making them fundamentally distinct from bacteria and warranting separate taxonomic classification
• PPLO are absolutely resistant to penicillin and require cholesterol as an essential nutrient, unlike bacteria or L-phase variants
• PPLO and bacterial L-phase variants are distinct groups that can be differentiated by colony morphology, filamentous structure formation, and metabolic requirements, contradicting earlier theories of shared origins
• The genus Mycoplasma is proposed as the legitimate generic name for all pleuropneumonia-like organisms, with fifteen established species identified and representative strains designated
• The order Mollicutales (based on the organisms' plastic, cell-wall-lacking nature) is recommended, with single family Mycoplasmataceae encompassing both parasitic and saprophytic species