Mollicutes: the wall-less bacteria (Mycoplasma, Spiroplasma, Phytoplasma)

Key characteristics

Mollicutes share several distinctive features that set them apart from most other bacteria. They are among the smallest bacteria in size (typically around 0.2–0.3 μm) and possess very compact genomes, making them models for minimal cellular life. Their defining traits include:

• Absence of a bacterial cell wall — a central diagnostic trait (no cell wall) that affects susceptibility to many antibiotics.
• Small cell and genome size; some species have genomes among the smallest known for free-living organisms (reduced genomes).
• Pleomorphism: cells are flexible and can assume diverse shapes because they lack a rigid wall.
• Varied motility: many glide along host surfaces (gliding), while certain groups such as Spiroplasma are helical and move by twisting.

Ecology, hosts and transmission

Most mollicutes are parasitic or commensal, living on or within the cells of animals or plants. In humans and other animals, genera such as Mycoplasma and Ureaplasma adhere tightly to mucosal surfaces and can cause respiratory, urogenital or systemic infections. For example, some species colonize the respiratory tract or the urethra (urethra) and may be associated with chronic or acute disease.

Plant-associated mollicutes include Phytoplasma and Spiroplasma, which are obligate pathogens of plants. These plant parasites are typically transmitted by sap-sucking insects; insect vectors such as leafhoppers and related Homoptera carry mollicutes between plants and introduce them when feeding (insect vectors).

Taxonomy and notable genera

The class contains several genera that differ in host range and biology. Mycoplasma and Ureaplasma are prominent in mammalian hosts; Spiroplasma includes helical, motile species often found in insects and plants; Phytoplasma are cell-wall-less bacteria restricted to plant phloem and insect vectors. Because mollicutes are fastidious and can be hard to culture, molecular methods are commonly used for detection and classification.

Medical and practical significance

Mollicutes are clinically important for several reasons. Their lack of a cell wall renders beta-lactam antibiotics (which target peptidoglycan synthesis) ineffective; treatment usually relies on macrolides, tetracyclines or fluoroquinolones selected according to species and resistance patterns. In agriculture, phytoplasma and spiroplasma infections can cause major crop losses and require integrated pest and vector management. In research, mollicutes’ reduced genomes have made them subjects of minimal genome studies and synthetic biology.

Laboratory notes and prevention

Because they are small, pleomorphic and require special nutrients (including sterols for many species), mollicutes are challenging to culture. Diagnostic approaches commonly combine serology, culture on enriched media, and nucleic acid tests. Control measures depend on the context: infection control and appropriate antibiotic selection in medicine, and vector control and resistant varieties in plant protection.

For further introductory material and taxonomic resources, consult general bacteriology and infectious disease references or curated online summaries (class overview, Mycoplasma pages, phytoplasma information and technical reviews on reduced genomes). Additional applied guidance on diagnosis and management may be found through medical and agricultural extension resources (cell-wall absence, motility mechanisms, clinical sites, genus summaries, vector control).