Bacillus subtilis is a rod-shaped, Gram-positive bacterium roughly two to four micrometers long and about a micrometer wide, but that simple description barely scratches the surface. This organism can radically reshape itself depending on conditions: assembling intricate multi-layered spores when starved, stretching into long filaments under stress, shedding its cell wall entirely to become a spherical L-form, or embedding itself in a complex biofilm matrix. Understanding how B. subtilis maintains its default rod shape, what structures decorate or support it, and how it survives conditions that would destroy most cells requires looking at several interacting systems at once.
How the Rod Shape Is Built and Maintained
The defining feature of a growing B. subtilis cell is its rod shape, and that shape comes from the peptidoglycan layer, a mesh of sugar chains cross-linked by short peptide bridges that wraps around the cell like a corset. Atomic force microscopy has shown that the glycan strands in this mesh can stretch up to five micrometers long, which is actually longer than the cell itself. On the inner surface of the wall, these strands are organized into cables roughly 50 nanometers wide, running mostly perpendicular to the cell’s long axis.1PubMed Central. Cell wall peptidoglycan architecture in Bacillus subtilis That circumferential arrangement is what gives the cell its elongated form rather than making it spherical.
Keeping the rod shape is not a passive affair. B. subtilis uses protein complexes called Rod complexes to lay down new peptidoglycan in an ordered, directional way. A second synthase called PBP1 adds peptidoglycan more randomly in all directions. The balance between these two modes of construction is critical. When the Rod complexes dominate, you get orderly elongation. When PBP1 takes over, the cell starts growing in a shapeless, amorphous way.2PubMed Central. Wall teichoic acids regulate peptidoglycan synthesis to maintain rod shape in Bacillus subtilis Mutations that upset this balance produce cells with severe shape defects, but the bacteria can sometimes evolve compensatory mutations that dial PBP1 back down, restoring normal morphology.3PubMed Central. Cell morphology maintenance in Bacillus subtilis through balanced peptidoglycan synthesis and hydrolysis
Wall Teichoic Acids as Shape Guardians
Threaded through the peptidoglycan are polymers called wall teichoic acids, long-chain molecules anchored in the cell wall. These are not just passive fillers. When teichoic acid production is shut down, the cell wall develops nanoscale pores within minutes. Those pores trigger a surge in PBP1 activity while stalling the Rod complexes, and the cell quickly loses its rod shape and becomes round.2PubMed Central. Wall teichoic acids regulate peptidoglycan synthesis to maintain rod shape in Bacillus subtilis Depleting teichoic acid biosynthesis proteins leads to a coccoid (ball-shaped) morphology and thickened cell walls, confirming these polymers are essential for normal shape.4PubMed Central. Teichoic acid is an essential polymer in Bacillus subtilis that is functionally distinct from teichuronic acid In effect, wall teichoic acids act as plugs that keep the cell wall intact and prevent the wrong type of growth machinery from taking over.
The Bacterial Cytoskeleton
Bacteria were once thought to lack cytoskeletons, but B. subtilis proved otherwise. A protein called MreB, structurally related to the actin found in animal cells, forms filaments that run along the inner surface of the cell membrane. Rather than sitting still, MreB rotates around the cell’s circumference, and that rotation depends on active cell wall synthesis.5PubMed Central. RodZ links MreB to cell wall synthesis to mediate MreB rotation and robust morphogenesis A related protein called Mbl forms helical cables inside the cell that are thought to direct where new peptidoglycan gets inserted. The model is that these helical tracks guide peptidoglycan insertion in a spiral pattern. As new wall material stretches along the cell’s long axis during growth, it generates torsional stress that actually causes the outer layers of the cell wall to twist in the opposite direction.6Developmental Cell. The Bacterial Cytoskeleton: In Vivo Dynamics of the Actin-like Protein Mbl of Bacillus subtilis
When MreB is absent or replaced with versions from distantly related bacteria, cells lose their rod shape.7PubMed. Influence of heterologous MreB proteins on cell morphology of Bacillus subtilis So the cytoskeleton is not just scaffolding; it actively steers the wall-building machinery to produce the right geometry.
Dividing the Cell
When B. subtilis is ready to divide, it uses another cytoskeletal protein: FtsZ, a distant relative of the tubulin found in animal cells. FtsZ monomers polymerize into a ring-like structure at the cell’s midpoint, called the Z-ring, which recruits the enzymes that build the division septum. A protein called FtsA interacts with FtsZ even before the ring assembles and is critical for forming a functional ring. Without FtsA, FtsZ still localizes at regular intervals along the cell, but most of the resulting rings are abnormal.8PubMed Central. Cell division in Bacillus subtilis: FtsZ and FtsA association is Z-ring independent, and FtsA is required for efficient midcell Z-Ring assembly
Researchers have pushed the system to find the minimum division machinery needed. By systematically deleting conserved division genes, one group managed to remove eight of them and found that only FtsZ and its membrane anchor SepF were strictly required for Z-ring formation and cell division.9PLoS Genetics. Minimization of the Bacillus subtilis divisome suggests FtsZ and SepF can form an active Z-ring, and reveals the amino acid transporter BraB as a new cell division influencing factor That minimalist setup only works with certain compensatory mutations in place, but it reveals how much of the division apparatus serves fine-tuning and quality-control roles rather than being mechanically essential.
Flagella and Swarming
Growing B. subtilis cells are motile. They swim using peritrichous flagella, meaning the filaments sprout from multiple points around the cell body rather than from one pole. On solid surfaces, the cells can also swarm, a collective form of movement where densely packed groups of bacteria glide outward across an agar surface. The leading edge of a swarm is made up of multicellular rafts of heavily flagellated cells. Both flagellum production and secretion of the lipopeptide surfactin are required for swarming to occur.10PubMed. Swarming motility in undomesticated Bacillus subtilis Surfactin lowers the surface tension of the liquid film on the agar, making it physically possible for the cell rafts to slide forward.
Competence Pili and DNA Uptake
Under certain conditions, a fraction of B. subtilis cells become “competent,” meaning they can take up naked DNA from their environment. This is not a passive process. Competent cells assemble a true pilus structure, a protein filament that extends outward from the cell surface, with an average length of about 500 nanometers. These pili can form at various locations around the cell and are used to grab environmental DNA and pull it across the cell wall.11PubMed Central. Uptake of environmental DNA in Bacillus subtilis occurs all over the cell surface through a dynamic pilus structure The pilus is dynamic, assembling and retracting, which helps transport the DNA inward through the thick peptidoglycan layer.
Intercellular Nanotubes
B. subtilis cells that are touching each other can share cytoplasmic contents through tubular extensions called nanotubes. Electron microscopy has revealed these as membrane tubes of varying diameters that bridge neighboring cells and allow the transfer of proteins and other molecules between them.12PubMed. Intercellular nanotubes mediate bacterial communication The exchange occurs in a spatially ordered manner, creating a network of conduits through a local population of cells. This is a fundamentally different mode of cell-to-cell communication from secreting signaling molecules into the surrounding medium; the molecules travel directly from one cell’s interior to another’s, keeping the exchange private and efficient.
Sporulation and the Morphological Transformation
The most dramatic structural change B. subtilis undergoes is sporulation, the process of forming an endospore. When nutrients run low, the cell divides asymmetrically near one pole, producing a small forespore and a large mother cell. The mother cell then engulfs the forespore in a process that resembles phagocytosis in animal immune cells.13PubMed Central. Cell-wall remodeling drives engulfment during Bacillus subtilis sporulation Engulfment relies on peptidoglycan remodeling enzymes (SpoIID, SpoIIM, and SpoIIP) that chew through the septal wall, allowing the mother cell membrane to creep forward around the forespore. A backup mechanism exists as well: zipper-like interactions between the forespore protein SpoIIQ and its partner SpoIIIAH on the mother cell side can drive engulfment when the primary machinery is weakened.14PubMed Central. Forespore engulfment mediated by a ratchet-like mechanism
Once engulfment is complete, the forespore sits inside the mother cell surrounded by two membranes. The mother cell then deposits a specialized cortex of modified peptidoglycan and an elaborate multi-layered coat around the developing spore. When the coat is finished, the mother cell lyses and releases the mature spore.
Endospore Architecture
The finished spore is a remarkably complex structure for something made by a single-celled organism. Working from the outside in, the coat consists of an outermost amorphous layer (the crust), a rodlet layer, a honeycomb layer, a fibrous layer, a layer of nanodot-sized particles, a multilayer assembly, and finally the undercoat or basement layer closest to the cortex.15PLOS ONE. Architecture and Assembly of the Bacillus subtilis Spore Coat Each layer has a distinct ultrastructure visible under electron microscopy, and each contributes differently to the spore’s defenses.
The outermost crust layer varies between strains. Laboratory strain PY79 shows a clear, continuous crust. Natto strains (used in Japanese fermented soybean production) have a disrupted crust that rarely encircles the entire spore, and some natto strains lack crust material entirely.16PubMed Central. Contributions of crust proteins to spore surface properties in Bacillus subtilis These differences in surface architecture affect how spores interact with their environment, including how easily they adhere to surfaces and how resistant they are to chemical treatments.
What Makes the Spore Core So Tough
At the heart of the endospore is the core, a highly dehydrated compartment containing the cell’s DNA, ribosomes, and enzymes needed for eventual germination. The core is packed with dipicolinic acid (DPA), which makes up roughly a tenth of the spore’s dry weight.17PubMed Central. Role of dipicolinic acid in survival of Bacillus subtilis spores exposed to artificial and solar UV radiation DPA, chelated with calcium ions, plays a central role in the spore’s resistance to heat, UV radiation, hydrogen peroxide, and desiccation. Spores engineered to lack DPA show sharply reduced resistance to wet heat, dry heat, peroxide, and drying, though their UV resistance remains similar to normal spores when other DNA-protective proteins are present.18PubMed Central. Role of dipicolinic acid in resistance and stability of spores of Bacillus subtilis with or without DNA-protective alpha/beta-type small acid-soluble proteins
How DPA protects the core has been debated. One older idea held that water inside the spore core exists in a glass-like state, essentially frozen in place, and that this immobility explains the spore’s toughness. Neutron scattering experiments have largely refuted this: water dynamics inside the spore look similar to those in fully hydrated biological systems, not glass. Instead, DPA accumulation during sporulation appears to reduce the amplitude of molecular motions in the core through a different mechanism, essentially dampening the jostling of larger molecules rather than freezing water in place.19Scientific Reports. The molecular dynamics of bacterial spore and the role of calcium dipicolinate in core properties at the sub-nanosecond time-scale
Germination and Waking Up
Despite their extraordinary dormancy, spores are primed to spring back to life when conditions improve. Germination can be triggered by nutrient signals detected by receptor proteins embedded in the spore’s inner membrane. It can also be triggered physically: moderate pressure (around 100 megapascals) activates those same germinant receptors, while much higher pressure (around 550 megapascals) bypasses the receptors entirely and forces open channels that release DPA from the core. That DPA release then triggers the downstream steps of germination, including cortex degradation and rehydration.20PubMed Central. Mechanisms of induction of germination of Bacillus subtilis spores by high pressure This pressure-induced pathway is relevant to the food industry, where high-pressure processing is used to kill or germinate spores in products that cannot be autoclaved.
Biofilm Architecture and Cellular Division of Labor
When B. subtilis colonizes a surface, individual cells can assemble into biofilms, communities encased in a self-produced matrix of polysaccharides, proteins, and extracellular DNA (eDNA).21PubMed Central. Matrix Production and Sporulation in Bacillus subtilis Biofilms Localize to Propagating Wave Fronts The three-dimensional structure of these biofilms depends on interactions between the exopolysaccharide (EPS) and eDNA components. eDNA plays a greater role in the early phases of biofilm development, while EPS becomes more important during maturation, and together they create the interconnected scaffold that holds the community together.22PubMed Central. The exopolysaccharide-eDNA interaction modulates 3D architecture of Bacillus subtilis biofilm
Within these biofilms, genetically identical cells adopt different transcriptional states, creating subpopulations with distinct behaviors. Some cells produce surfactin, some become competent for DNA uptake, some begin sporulating, and some produce matrix components. Earlier models depicted this as clean differentiation into fixed “cell types,” but single-cell studies have shown the picture is messier. Many cells multitask, expressing genes from several programs simultaneously. Only a few subpopulations, like surfactin producers, sporulating cells, and competent cells, appear to have truly distinct roles.23PubMed Central. Extensive cellular multi-tasking within Bacillus subtilis biofilms
The relationship between biofilm formation and sporulation is also more nuanced than previously thought. Rather than biofilm being a stepping stone on a linear path to sporulation, the two developmental programs appear to branch apart. A subpopulation of cells that initiates sporulation breaks down a component of its own cell envelope (lipoteichoic acid) to release glycerol. That glycerol diffuses outward and acts as a chemical signal directing neighboring cells to form biofilms instead of spores, essentially creating a branched developmental pathway where one cell fate actively generates the other.24PubMed Central. A metabolite morphogen coordinates multicellular development in Bacillus subtilis
Cannibalism as a Survival Strategy
Before fully committing to sporulation, B. subtilis employs a strikingly aggressive delay tactic. Cells that have begun the sporulation program produce two toxin systems, Skf and Sdp, that kill their non-sporulating siblings. The lysed cells release nutrients back into the starving community, which the sporulating cells use to keep growing. Because sporulation is costly and irreversible once completed, this cannibalism effectively buys the sporulating cells more time, allowing them to halt spore development and resume growth if conditions improve.25PubMed Central. Cannibalism enhances biofilm development in Bacillus subtilis The toxins kill cooperatively, and the nutrients released feed the broader community, linking cannibalism to biofilm development as well.26FEMS Microbiology Reviews. Cannibalism: a social behavior in sporulating Bacillus subtilis
Filamentation Under Stress
When B. subtilis encounters DNA damage, it activates an SOS response that blocks cell division, causing cells to continue growing without dividing. The result is long, multinucleate filaments that can be many times the normal cell length. A protein called YneA is the SOS-induced division inhibitor that enforces this checkpoint, though exactly how it blocks division at a molecular level remains unclear.27PubMed Central. DNA damage checkpoint activation affects peptidoglycan synthesis and late divisome components in Bacillus subtilis Filamentation is thought to give the cell time to repair its DNA before attempting to partition chromosomes into daughter cells, where unrepaired breaks could be lethal.
Wall-Free L-Forms
Perhaps the most extreme morphological state B. subtilis can adopt is the L-form, a cell that has completely lost its peptidoglycan wall. L-forms are spherical, irregularly shaped, and can only survive in osmotically stabilized media that prevent them from bursting. Researchers have engineered B. subtilis strains that can be switched between normal rod-shaped growth and L-form growth on demand.28PubMed Central. Cell envelope stress response in cell wall-deficient L-forms of Bacillus subtilis L-forms are fascinating because they abandon the FtsZ-based division machinery entirely. Instead, they proliferate through a purely biophysical process driven by an imbalance between membrane surface area and cell volume, essentially blebbing and pinching off daughter cells without any dedicated division apparatus.29Current Biology. Cell Growth of Wall-Free L-Form Bacteria Is Limited by Oxidative Damage L-forms are inherently resistant to antibiotics that target the cell wall, since they have none, which has made them a subject of interest in understanding how bacteria might evade certain drug classes.
Membrane Tuning in Cold Environments
While the cell wall provides structural rigidity, the plasma membrane must stay fluid enough to function across a range of temperatures. B. subtilis adjusts its membrane composition using two strategies. For long-term cold adaptation, it shifts the ratio of its branched-chain fatty acids, increasing the proportion of anteiso-branched fatty acids (which have lower melting points) relative to iso-branched ones.30Biochimica et Biophysica Acta (BBA) – Biomembranes. Metabolic control of the membrane fluidity in Bacillus subtilis during cold adaptation For rapid response after a sudden cold shock, it activates a fatty acid desaturase enzyme that introduces double bonds into the fatty acids already present in membrane phospholipids, quickly lowering the temperature at which the membrane would solidify. The ratio of branched to straight-chain fatty acids stays roughly constant regardless of temperature; what changes is the type of branching.31PubMed. Unsaturated and branched chain-fatty acids in temperature adaptation of Bacillus subtilis and Bacillus megaterium
Lab Strains Versus Wild Strains
A wrinkle that affects almost everything we know about B. subtilis morphology is domestication. The most widely used laboratory strain, B. subtilis 168, has been cultured in labs since the 1940s and has accumulated mutations that make it behave differently from wild isolates. Compared to the wild-type strain NCIB 3610, strain 168 forms drastically weaker biofilms. Tracing the genetic basis for this difference revealed that mutations in at least four genes (sfp, epsC, swrA, and degQ) are jointly responsible. The wild strain also carries a large plasmid, absent in 168, that encodes a gene called rapP which influences biofilm architecture.32PubMed Central. Tracing the domestication of a biofilm-forming bacterium These are not trivial differences. Wild-type B. subtilis 3610 also outperforms strain 168 in practical applications like biosynthesizing nanoparticles, reflecting how deeply domestication reshapes the organism’s metabolic and structural capabilities.33PubMed Central. Strain-Specific Bacillus subtilis-Derived Silver Nanoparticles for Effective Antibacterial Activity Against Multidrug-Resistant Pathogens: In Vitro Model Researchers studying B. subtilis morphology or community behavior increasingly use undomesticated strains or at least acknowledge that results from 168 may not reflect the full repertoire of structures and behaviors the species is capable of.