Mycoplasmatota

Mycoplasmatota (formerly Tenericutes, from the Latin tener, "soft / tender," and cutis, "skin," or class Mollicutes, "soft skin") is a lineage of bacteria characterized by the complete absence of a peptidoglycan cell wall and outer membrane, bounding their cytoplasm solely with a single, cholesterol-fortified trilaminar plasma membrane. Although historically classified as an independent phylum based on their pleomorphic, filterable, wall-less morphology, phylogenomic analyses demonstrate that Mycoplasmatota evolved via extreme reductive evolution from a low-G+C Gram-positive ancestor nested inside or sister to Bacillota (Firmicutes).

Because they lack peptidoglycan and penicillin-binding proteins, all members of Mycoplasmatota are intrinsically resistant to β-lactam, glycopeptide, fosfomycin, and bacitracin antibiotics, exhibit a characteristic "fried-egg" colony morphology on agar, and occupy obligate commensal or parasitic niches across vertebrates, arthropods, and phloem tissues of vascular plants:

1. Minimal-Genome Model Organisms and Synthetic Biology: Mycoplasma genitalium possesses the smallest genome of any known self-replicating free-living/axenically culturable organism (580,070 bp, encoding only 470–482 protein-coding genes), and Mycoplasma mycoides served as the recipient template for J. Craig Venter and Hamilton Smith's synthetic minimal cell JCVI-syn3.0 (531 kb, 473 genes).
2. Human Mucosal Pathogens: Mycoplasma pneumoniae (the leading cause of community-acquired "walking" atypical pneumonia), Mycoplasma genitalium, Mycoplasma hominis, and urease-positive Ureaplasma urealyticum / U. parvum.
3. Insect-Vectored Plant Pathogens (Spiroplasma and Phytoplasma): Helical, motile Spiroplasma (citrus stubborn and corn stunt diseases; Drosophila male-killing endosymbionts) and unculturable phloem-limited Candidatus Phytoplasma (aster yellows, witches'-broom, and lethal yellowing of palms).

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` Phylum Mycoplasmatota / Class Mollicutes (Reductive clade derived from ancestral Bacillota; 20–40% G+C) ├── Acholeplasmatales / Anaeroplasmatales Clade (Retain ancestral UGA = Stop; sterol-independent or anaerobic) │ ├── Acholeplasmataceae (Acholeplasma laidlawii: synthesizes carotenoid membrane reinforcers instead of requiring sterols) │ └── Candidatus Phytoplasma (Obligate phloem-sieve-element plant pathogens vectored by Hemiptera leafhoppers/psyllids) │ └── Core UGA = Tryptophan Reassigned Clade (Sterol auxotrophs) ├── Entomoplasmatales (Spiroplasma citri, S. kunkii, S. poulsonii, Mesoplasma florum: helical wall-less swimmers) ├── Mycoplasmoidales / Hominis & Pneumoniae Groups │ ├── Pneumoniae Group (Mycoplasma pneumoniae [816 kb], M. genitalium [580 kb], M. gallisepticum, M. mobile) │ ├── Hominis Group (Mycoplasma hominis, M. hyorhinis, M. fermentans, M. pulmonis) │ └── Ureaplasma (Ureaplasma parvum, U. urealyticum: generate 95% of ATP via intracellular urea hydrolysis!) └── Mycoides Cluster (Spiroplasma phylogenetic group) └── Mycoplasma mycoides subsp. mycoides (Contagious bovine pleuropneumonia; chassis for JCVI-syn1.0 & JCVI-syn3.0)

As Mycoplasmatota transitioned to osmotically buffered mucosal surfaces, arthropod hemolymph, and plant phloem sap, relaxed purifying selection combined with genetic drift (Muller's ratchet) purged pathways for cell-wall synthesis, the tricarboxylic acid (TCA) cycle, cytochromes, and de novo synthesis of amino acids, purines, pyrimidines, and fatty acids:

• From the First Sequenced Genomes to JCVI-syn3.0: In 1995, Mycoplasma genitalium (580 kb) became the second cellular genome ever sequenced, launching the minimal-gene-set concept. In 2010, the J. Craig Venter Institute chemically synthesized the 1.08-Mb genome of Mycoplasma mycoides (JCVI-syn1.0) and transplanted it into an enucleated Mycoplasma capricolum cell. By iteratively deleting non-essential genes via global transposon mutagenesis (Hutchison et al., 2016), they engineered JCVI-syn3.0: a 531-kb synthetic chromosome containing only 473 genes (438 protein-coding genes + 35 RNA genes, of which 149 genes still had unknown biological functions at the time of synthesis) that divides autonomously every ~180 minutes.
• Reassignment of the Opal Stop Codon (UGA → Tryptophan): Driven by severe AT-bias (70–78% A+T content), the ancestor of the Mycoplasma–Ureaplasma–Spiroplasma clade completely lost peptide chain release factor 2 (prfB / RF2) (which normally terminates translation at UGA and UAA) while evolving a mutant tRNA^Trp carrying a UCA anticodon that pairs with both UGG and UGA. Consequently, any Mycoplasma gene containing internal UGA tryptophan codons prematurely truncates if expressed directly in Escherichia coli or human cells—acting as a natural genetic firewall against horizontal gene transfer.

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1. Cholesterol-Fortified Plasma Membranes: To prevent osmotic lysis in the absence of a peptidoglycan wall, Mycoplasma, Ureaplasma, and Spiroplasma scavenge cholesterol (or plant β-sitosterol/stigmasterol) directly from host cell membranes via high-affinity surface lipoproteins, incorporating sterols up to 35–50 mol% of total membrane lipid. This sterol dependence renders mycoplasmas sensitive to polyene macrolides (amphotericin B, filipin) and digitonin, while leaving them completely immune to penicillins, cephalosporins, and carbapenems.
2. Urea-Hydrolysis Chemiosmotic ATP Synthesis in Ureaplasma: Ureaplasma parvum and U. urealyticum lack both glycolysis and arginine deiminase pathways. Instead, they generate >95% of their cellular ATP via a unique urea-driven chemiosmotic mechanism:
   • Urea imported from host urogenital fluids is hydrolyzed inside the cytoplasm by a potent nickel-dependent urease into carbon dioxide and two molecules of ammonia (CO(NH₂)₂ + H_2O → CO₂ + 2NH₃).
   • Intracellular protonation of NH₃ to NH₄⁺ alkalinizes the cytoplasm and generates an electrochemical ammonium/proton potential (Δ p) across the plasma membrane that directly drives the F₁F_O-ATP synthase.

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Despite lacking bacterial flagella, peptidoglycan, and MreB (in most species), three distinct lineages of Mycoplasmatota independently evolved complex, non-homologous cytoskeletal motility machines from repurposed housekeeping enzymes:

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• Mycoplasma pneumoniae and the CARDS Toxin: Attaching specifically to sialylated oligosaccharides at the base of human respiratory epithelial cilia via its P1 adhesin terminal organelle, M. pneumoniae halts mucociliary clearance (ciliostasis), generates localized hydrogen peroxide (H_2O2) and superoxide via glycerol-3-phosphate oxidase (*GlpD`), and secretes the Community-Acquired Respiratory Distress Syndrome (CARDS) toxin—an ADP-ribosylating and vacuolating protein homologous to the S1 subunit of Bordetella pertussis toxin that activates the host NLRP3 inflammasome. Cross-reactive host IgM antibodies targeting sialylated I-antigen glycolipids on human erythrocytes frequently induce cold agglutinin hemolytic anemia.
• Phytoplasma Effectors (SAP54 and SAP11) and Floral "Zombie" Reprogramming: Inside plant phloem sieve tubes, Candidatus Phytoplasma secretes small effector proteins through Sec translocons that move systemically through plasmodesmata into shoot apical meristems:
  • SAP54 binds host MADS-box floral transcription factors and bridges them to the 26S proteasome shuttle protein RAD23 for ubiquitin-independent degradation, converting flowers into sterile, leafy green shoots (phyllody and virescence).
  • SAP11 destabilizes TCP transcription factors, suppressing jasmonic acid (JA) plant defenses and inducing bushy "witches'-broom" axillary shoot proliferation—turning the infected plant into an attractive, non-reproducing green feeding platform ("zombie plant") that maximizes reproduction of the phytoplasma's leafhopper insect vector!

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1. Fraser, C. M., et al. (1995). The minimal gene complement of Mycoplasma genitalium. Science, 270(5235), 397–403. Google Scholar
2. Hutchison, C. A., et al. (2016). Design and synthesis of a minimal bacterial genome. Science, 351(6280), aad6253. Google Scholar
3. Miyata, M., Robinson, R. C., Uyeda, T. Q., Fukumori, Y., Fukushima, S. I., Haruta, S., ... & Tulum, I. (2020). Tree of motility: a proposed history of motility systems in the tree of life. Genes to Cells, 25(1), 6–21. Google Scholar
4. MacLean, A. M., Orlovskis, Z., Kowitwanich, K., Zdziarska, A. M., Angenent, G. C., Immink, R. G., & Hogenhout, S. A. (2014). Phytoplasma effector SAP54 hijacks plant reproduction by degrading MADS-box proteins and promotes insect colonization in a RAD23-dependent manner. PLOS Biology, 12(4), e1001835. Google Scholar

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• Bacillota
• Microbiome
• Hemiptera
• Synthetic Biology
• Antibiotic Resistance