Bacillota

Bacillota (formerly Firmicutes, from the Latin firmus, "strong," and cutis, "skin," referring to their thick, mechanically rigid peptidoglycan cell wall) is a major phylum of low-G+C Gram-positive (predominantly monoderm) bacteria within the Terrabacteria superclade. Whereas their sister Gram-positive phylum Actinomycetota exhibits high genomic guanine-plus-cytosine content (>50% G+C), most Bacillota possess AT-rich genomes ranging from 24% to 50% G+C (with rare exceptions such as Geobacillus and Symbiobacterium).

Bacillota encompasses an extraordinary metabolic and ecological spectrum across aerobic, facultatively anaerobic, and strictly anaerobic niches:

1. Endospore-Forming Bacilli and Clostridia (Bacillus, Paenibacillus, Clostridium, Clostridioides, Moorella): Lineages capable of asymmetric cell division to construct metabolically dormant endospores—the hardiest biological structures on Earth, able to survive boiling (100 °C), vacuum desiccation, ultraviolet and γ-radiation, organic solvents, and multi-century dormancy.
2. The Lactic Acid Bacteria (Order Lactobacillales): Non-spore-forming fermentative rods and cocci (Lactobacillus, Lacticaseibacillus, Lactococcus, Leuconostoc, Pediococcus, Enterococcus, and Streptococcus) that drive global food fermentation (yogurt, cheese, sourdough, sauerkraut, kimchi), colonize human mucosal surfaces, and include major invasive pathogens (Streptococcus pneumoniae, S. pyogenes).
3. Staphylococci and Listeriae (Staphylococcus aureus, Listeria monocytogenes): Halotolerant skin commensals and opportunistic clinical pathogens (MRSA) alongside psychrotolerant intracellular foodborne bacteria that hijack host actin polymerization.
4. Dominant Human Gut Microbiome Symbionts (Orders Eubacteriales / Lachnospirales and Oscillospirales): Obligate anaerobic Clostridia (Faecalibacterium prausnitzii, Roseburia, Agathobacter, Ruminococcus) that constitute 30–60% of the healthy adult human colonic microbiota and ferment dietary fiber into the anti-inflammatory short-chain fatty acid butyrate.

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` Phylum Bacillota (Firmicutes; >3,500 described species) ├── Class Bacilli (Predominantly aerobic or facultatively anaerobic rods and cocci) │ ├── Order Caryophanales / Bacillales │ │ ├── Bacillaceae (Bacillus subtilis [model Gram-positive], B. anthracis, B. cereus, B. thuringiensis, Geobacillus) │ │ ├── Paenibacillaceae (Paenibacillus polymyxa [polymyxin antibiotic & dinitrogen fixation], P. larvae) │ │ ├── Staphylococcaceae (Staphylococcus aureus, S. epidermidis) │ │ └── Listeriaceae (Listeria monocytogenes) │ └── Order Lactobacillales (Lactic Acid Bacteria — non-spore-forming, cytochrome-deficient fermenters) │ ├── Lactobacillaceae (Lactobacillus acidophilus, Lacticaseibacillus rhamnosus, Lactiplantibacillus plantarum, Leuconostoc) │ ├── Streptococcaceae (Lactococcus lactis, Streptococcus pneumoniae, S. pyogenes, S. mutans) │ └── Enterococcaceae (Enterococcus faecalis, E. faecium [VRE]) │ ├── Class Clostridia (Strictly anaerobic rods, cocci, and gut commensals) │ ├── Order Eubacteriales / Clostridiales (Clostridium botulinum, C. tetani, C. perfringens, C. acetobutylicum) │ ├── Order Peptostreptococcales (Clostridioides difficile [formerly Clostridium difficile]) │ ├── Order Lachnospirales (Roseburia intestinalis, Blautia, Butyrivibrio) │ ├── Order Oscillospirales (Faecalibacterium prausnitzii, Ruminococcus bromii) │ └── Order Thermoanaerobacterales (Moorella thermoacetica: Wood–Ljungdahl reductive acetogenesis) │ └── Derived / Specialized Lineages ├── Class Negativicutes (Veillonella, Selenomonas, Megasphaera: diderm Bacillota with an authentic LPS outer membrane!) ├── Class Erysipelotrichia (Erysipelothrix, Turicibacter) └── Class Tissierellia & Culicoidibacteria (Peptoniphilus, Finegoldia)

In typical monoderm Bacillota (Bacillus subtilis, Staphylococcus aureus), the cytoplasmic membrane is surrounded by a 20- to 80-nm-thick multilayered mesh of peptidoglycan (murein) accounting for 40–90% of the dry weight of the cell wall and withstanding internal osmotic turgor pressures of 15–25 atmospheres (1.5–2.5 MPa):

• Peptidoglycan Cross-Linking: Alternating chains of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) are cross-linked via stem peptides, either directly between meso-diaminopimelic acid (m-DAP) at position 3 and D-alanine at position 4 (in Bacillus and Clostridium) or across a flexible pentaglycine interpeptide bridge (Gly₅) synthesized by FemA/FemB/FemX transferases (in Staphylococcus aureus, the target of lysostaphin and β-lactam antibiotics).
• Wall Teichoic Acids (WTAs) and Lipoteichoic Acids (LTAs): Interwoven throughout the peptidoglycan matrix are anionic teichoic acids—linear polymers of glycerol phosphate or ribitol phosphate representing up to 50% of cell-wall mass. Wall teichoic acids (WTAs) are covalently linked via phosphodiester bonds to the C6 hydroxyl of MurNAc, whereas lipoteichoic acids (LTAs) are anchored via a glycolipid moiety into the outer leaflet of the plasma membrane. Their dense negative phosphate charges chelate divalent cations (Mg²⁺ and Ca²⁺), regulate endogenous autolysins (preventing self-rupture during cell division), and can be esterified with D-alanine (dltABCD operon) to neutralize negative surface charge and repel cationic host antimicrobial peptides (defensins) and glycopeptide antibiotics.

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When Bacillus or Clostridium cells encounter severe starvation, histidine sensor kinases (KinA–KinE) initiate a multicomponent phosphorelay (KinA → Spo0F → Spo0B → Spo0A) that phosphorylates the master transcription factor Spo0A~P, diverting the cell from symmetric binary fission to an 8-hour, 7-stage asymmetric morphogenesis governed by compartment-specific RNA polymerase sigma factors (σ^F and σ^G in the smaller forespore; σ^E and σ^K in the larger mother cell):

1. Asymmetric Septation and Engulfment: The cell divides near one pole into a smaller forespore and a larger mother cell. The mother cell membrane migrates around the forespore, completely engulfing it as a double-membrane-bound protoplast inside the mother cell cytoplasm before the mother cell eventually lyses to release the mature spore.
2. Core Dehydration and Calcium–Dipicolinic Acid (Ca-DPA): The mother cell synthesizes massive amounts of dipicolinic acid (pyridine-2,6-dicarboxylic acid, DPA) (spoVF operon), which is pumped into the forespore core via the SpoVA channel in a 1:1 chelate with divalent calcium (Ca²⁺–DPA). Ca-DPA accumulates to 10–15% of total spore dry weight, displacing free water (reducing core water content to 25–50% of wet weight) and transitioning the spore cytoplasm into an immobilized, high-viscosity glassy state (vitrification) that prevents thermal protein denaturation up to 100–120 °C (requiring pressurized steam autoclaving at 121 °C for 15–20 minutes for sterilization).
3. Small Acid-Soluble Spore Proteins (SASPs): The forespore expresses α/β-type Small Acid-Soluble Spore Proteins (SASPs) that saturate the spore's chromosome, clamping the DNA from its normal B-form into an A-like helical conformation. This conformational shift suppresses cytosine deamination, blocks depurination, and prevents UV photons (254 nm) from forming mutagenic cyclobutane pyrimidine dimers (redirecting UV photochemistry to the harmless "spore photoproduct" 5-thyminyl-5,6-dihydrothymine, which is immediately repaired upon germination by the radical-SAM enzyme spore photoproduct lyase, SplB).
4. Cortex and Multilayered Protein Coat: Surrounding the inner spore membrane is the germ cell wall and a thick cortex of modified peptidoglycan in which 50% of MurNAc residues are converted to muramic-δ-lactam, encased inside a cross-linked disulfide/dityrosine protein coat (>70 proteins) that excludes lysozyme, hydrogen peroxide, and predatory protozoan digestive enzymes. Upon sensing nanomolar nutrient germinants (L-alanine, inosine, or primary bile salts such as taurocholate in Clostridioides difficile) via inner-membrane Ger receptors, the spore releases its Ca-DPA depot, activates cortex-lytic enzymes (SleB / CwlJ), rehydrates within minutes, and resumes vegetative growth.

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1. Colonic Butyrate Synthesis and Gut Homeostasis: Within the anoxic human colon, commensal Clostridia in Clostridium clusters IV and XIVa (Faecalibacterium prausnitzii, Eubacterium rectale, Roseburia spp.) ferment resistant starch (Ruminococcus bromii amylosomes), inulin, and pectin via the acetyl-CoA → butyryl-CoA:acetate CoA-transferase pathway into butyrate (reaching 10–25 mM in the colonic lumen). Butyrate serves as the primary oxidative fuel (~70% of ATP) for human colonic epithelial cells (colonocytes), maintains mucosal hypoxia to exclude facultative anaerobic pathogens (Salmonella, E. coli), inhibits histone deacetylases (HDACs), and binds GPCR receptors (GPR41, GPR43, GPR109A) to induce anti-inflammatory colonic Foxp3⁺ regulatory T cells (T_regs).
2. Industrial ABE Fermentation and Agricultural Biological Control:
   • Clostridium acetobutylicum (the Weizmann organism) performs biphasic Acetone–Butanol–Ethanol (ABE) fermentation, converting agricultural starches and lignocellulosic sugars into industrial biofuel n-butanol and acetone.
   • Bacillus thuringiensis (Bt): During sporulation, Bt synthesizes a parasporal crystalline inclusion composed of δ-endotoxins (Cry and Cyt proteins). When ingested by susceptible lepidopteran, coleopteran, or dipteran insect larvae, alkaline midgut proteases cleave the protoxin into an active core that binds cadherin and aminopeptidase-N receptors, forming lytic pores in the midgut epithelium—forming the basis of both organic Bt microbial sprays and transgenic Bt cotton and Bt maize.
3. Potent Protein Exotoxins of Pathogenic Bacillota:
   • Clostridium botulinum and Clostridium tetani: Synthesize zinc-metalloprotease botulinum neurotoxins (BoNT/A–G) and tetanospasmin (TeNT)—the most potent biological toxins known (human lethal dose ~1 ng/kg). Both cleave SNARE proteins (SNAP-25, synaptobrevin/VAMP, or syntaxin) inside synaptic terminals: BoNT blocks acetylcholine exocytosis at peripheral neuromuscular junctions (causing flaccid paralysis and clinical botulism, while purified picogram doses of BoNT/A are used therapeutically to treat dystonia, spasticity, and chronic migraine), whereas TeNT undergoes retrograde axonal transport to spinal cord Renshaw interneurons to block glycine/GABA release (causing spastic paralysis / lockjaw).
   • Bacillus anthracis: Carries two virulence plasmids: pXO2 (encoding an antiphagocytic poly-γ-D-glutamic acid polypeptide capsule) and pXO1 (encoding the tripartite anthrax toxin: Protective Antigen [PA], Edema Factor [EF, a calmodulin-dependent adenylate cyclase], and Lethal Factor [LF, a zinc metalloprotease that cleaves MAPK kinases MEK1–MEK7]).
   • Clostridioides difficile: Following broad-spectrum antibiotic disruption of commensal gut Clostridia (which normally convert primary taurocholate bile acids into germination-inhibiting secondary bile acids such as deoxycholate and lithocholate via *bai` 7α-dehydroxylase operons), C. difficile spores germinate and secrete the large Rho-glucosylating toxins TcdA and TcdB, disrupting the enterocyte actin cytoskeleton and causing pseudomembranous colitis.

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1. Kunst, F., et al. (1997). The complete genome sequence of the Gram-positive bacterium Bacillus subtilis. Nature, 390(6657), 249–256. Google Scholar
2. Setlow, P. (2014). Germination of spores of Bacillus species: what we know and do not know. Journal of Bacteriology, 196(7), 1297–1305. Google Scholar
3. Louis, P., & Flint, H. J. (2017). Formation of propionate and butyrate by the human colonic microbiota. Environmental Microbiology, 19(1), 29–41. Google Scholar
4. Poulain, B., & Popoff, M. R. (2019). Why are botulinum neurotoxin-producing bacteria so diverse and botulinum neurotoxins so toxic? Toxins, 11(1), 34. Google Scholar%2C%2034)

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• Actinomycetota
• Microbiome
• Antibiotic Resistance
• Fermentation
• Cell Wall