7.17: Bacterial Phylum Tenericutes

The phylum Tenericutes, which includes the single class Mollicutes, comprises bacteria that lack cell walls. The term "Mollicutes" derives from the Latin word mollis, meaning "soft." These organisms are among the smallest known and are commonly referred to as mycoplasmas due to the prominence of the genus Mycoplasma, which includes well-known human pathogens. Despite their inability to stain gram-positively (a result of their lack of cell walls), mycoplasmas are phylogenetically related to the Firmicutes.

Mycoplasmas often live in close association with animal or plant hosts, where the absence of a gram-positive cell wall may be an advantage. They have small genomes, ranging from 0.6 to 2.2 megabase pairs (Mbp), a feature typical of obligate symbionts.

Structural Properties

The absence of cell walls in mycoplasmas has been confirmed through electron microscopy and chemical analyses, which reveal a lack of peptidoglycan. While they resemble protoplasts (bacteria stripped of their cell walls), mycoplasmas are more resistant to osmotic lysis. This resilience is partly due to the presence of sterols, which stabilize their cytoplasmic membrane. Some mycoplasmas require sterols in their growth media, a feature useful in their classification.

In addition to sterols, many mycoplasmas possess lipoglycans—long-chain heteropolysaccharides covalently bound to membrane lipids. These compounds, embedded in the cytoplasmic membrane, resemble the lipopolysaccharides of gram-negative bacteria but lack the lipid A backbone. Lipoglycans contribute to membrane stability and facilitate attachment to animal cell surface receptors.

Mycoplasmas exhibit pleomorphic forms, with cultures often showing small coccoid cells (0.2–0.3 μm), swollen cells, and filamentous, branched forms. In agar cultures, colonies exhibit a characteristic "fried egg" appearance with a dense central core embedded in the medium and a lighter surrounding area. Unlike bacteria with cell walls, mycoplasmas are unaffected by antibiotics targeting cell wall synthesis but are susceptible to antibiotics targeting other cellular processes.

Mycoplasmas are cultivated on complex media, often requiring fresh serum or ascitic fluid to provide sterols and unsaturated fatty acids. While some species grow poorly even on rich media, others can thrive on defined media. Most mycoplasmas use carbohydrates for energy and require vitamins, amino acids, purines, and pyrimidines. Their energy metabolism varies, with species exhibiting aerobic, facultative aerobic, or anaerobic growth.

Spiroplasma, a genus of helical or spiral-shaped Mollicutes, lacks both cell walls and flagella yet exhibits motility through rotary (screw-like) motion or slow undulations. This motility is believed to involve intracellular fibrils. Spiroplasma species are found in diverse environments, including ticks, the hemolymph and gut of insects, plant vascular fluids, and on plant surfaces such as flowers and leaves.

One notable species, Spiroplasma citri, causes citrus stubborn disease and has also been linked to corn stunt disease. Other plant-associated Mollicutes have been identified through electron microscopy, suggesting the existence of a large, diverse group. Additionally, some Spiroplasma species cause insect diseases, such as honeybee spiroplasmosis and lethargy disease in beetles like Melolontha.