Mollicutes

Mollicutes are a class of bacteria characterized by the absence of a cell wall, small size, and greatly reduced genomes. They include genera such as Mycoplasma and Ureaplasma, and are among the smallest free-living or parasitic bacteria. Their membranes contain sterols, which helps stabilize the cell in the absence of a rigid cell wall. Because they lack peptidoglycan, Mollicutes typically stain poorly with standard Gram staining and exhibit pleomorphism.

Biology and ecology

Most Mollicutes are parasites or commensals that depend on their hosts for nutrients. They are fastidious and often require rich, specialized culture media that include sterols (cholesterol) and complex nutrients. Their genomes are among the smallest in bacteria, reflecting reductive evolution associated with a parasitic or host-dependent lifestyle. In practice, many species are restricted to specific mammalian or plant hosts, though some plant-associated Spiroplasma show broader ecological roles.

Taxonomy and significance

The class Mollicutes is part of the broader group sometimes referred to as Tenericutes; taxonomy has varied in recent years. Major human-associated genera include Mycoplasma and Ureaplasma. Notable species include Mycoplasma pneumoniae, a cause of atypical pneumonia; Mycoplasma genitalium, linked to nongonococcal urethritis; and Ureaplasma urealyticum, associated with urogenital infections. Mollicutes are also important in veterinary medicine and plant pathology, where related species can cause diseases.

Culture, diagnosis, and treatment

Mollicutes are difficult to culture and require specialized media, often with serum or cholesterol. Because they lack a cell wall, they are insusceptible to beta-lactam antibiotics; treatment relies on agents that inhibit protein synthesis or DNA replication, such as macrolides, tetracyclines, or fluoroquinolones. Diagnosis commonly uses molecular methods (PCR) and culture in enriched media, supplemented by serology in certain contexts.