Bacterial Phylum Tenericutes

Mycoplasmas are members of the phylum Tenericutes, class Mollicutes, and have lost their cell walls entirely. This absence of peptidoglycan, confirmed through electron microscopy, distinguishes them from typical bacteria. The loss may provide advantages in their obligate symbiotic lifestyle with animal or plant hosts, allowing closer association with host cells.

Despite lacking cell walls, mycoplasmas resist osmotic lysis through sterols and lipoglycans embedded in their cytoplasmic membrane. Sterols stabilize the membrane, and many species require sterols in growth media for survival. Lipoglycans, long-chain heteropolysaccharides covalently bound to membrane lipids, further enhance membrane stability and facilitate attachment to host cell receptors.

Mycoplasmas exhibit pleomorphic forms, appearing as small coccoid cells (0.2–0.3 μm), large swollen cells, and filamentous branched forms. In agar cultures, they form characteristic fried egg colonies with a dense central core embedded in the medium and a lighter surrounding area. This morphological diversity reflects their flexible cell structure.

Spiroplasma, a helical or spiral-shaped genus within Mollicutes, lacks both cell walls and flagella yet exhibits motility through rotary screw-like motion or slow undulations. This movement is believed to involve intracellular fibrils that enable locomotion. Spiroplasma species inhabit diverse environments, from insect hemolymph and plant vascular fluids to flower and leaf surfaces.

Several mycoplasma species are pathogenic to humans and plants. Spiroplasma citri causes citrus stubborn disease and corn stunt disease in plants. Human pathogens include Mycoplasma pneumoniae, Ureaplasma urealyticum, and Metamycoplasma hominis. Some Spiroplasma species also cause insect diseases such as honeybee spiroplasmosis.

Mycoplasmas possess small genomes ranging from 0.6 to 2.2 megabase pairs, a feature typical of obligate symbionts. Their reduced genome size reflects their dependence on host cells for survival. Most species require complex media containing fresh serum or ascitic fluid to provide essential sterols, unsaturated fatty acids, vitamins, amino acids, and nucleotide precursors.

Mycoplasmas belong to the phylum Tenericutes, which contains a single class, Mollicutes, derived from Latin mollis meaning soft. Though they lack gram-positive cell walls, mycoplasmas are phylogenetically related to the bacterial phylum Firmicutes. Understanding their classification requires methods of classification and identification that account for their unique structural absence and molecular relationships.