Bacillus subtilis is firmly Gram-positive, and it has served as the defining model organism for the entire Gram-positive lineage for over four decades. Its thick peptidoglycan cell wall retains the crystal violet dye during the Gram staining procedure, producing the characteristic purple color that defines the classification. But the full story is more interesting than a one-word answer, because under certain conditions B. subtilis can actually lose its ability to hold that stain, and recent evolutionary research has complicated the neat divide between Gram-positive and Gram-negative bacteria altogether.
What Makes It Gram-Positive
The Gram stain works by exploiting a structural difference in bacterial cell walls. During the procedure, cells are flooded with crystal violet dye and then treated with iodine, which acts as a mordant to lock the dye in place. An alcohol wash follows. Bacteria with thin cell walls surrounded by an outer membrane (Gram-negative) lose the dye during that wash and pick up a pink counterstain instead. Bacteria with thick, multi-layered peptidoglycan walls and no outer membrane (Gram-positive) trap the dye-iodine complex inside the cell, staying purple.
In B. subtilis, the crystal violet–iodine complex forms both at the cell surface and within the cell interior. The alcohol decolorization step dissolves the surface precipitate, but the internal complex is physically retained by the thick peptidoglycan wall and stays put.1PubMed Central. Cellular responses of Bacillus subtilis and Escherichia coli to the Gram stain That retention is the whole basis of the Gram-positive designation. The wall is doing the work.
The Cell Wall That Does the Heavy Lifting
B. subtilis has one of the most thoroughly studied cell walls in microbiology. Its peptidoglycan layer is remarkably thick compared to Gram-negative species, and the glycan strands that form the scaffolding can stretch up to 5 micrometers long, which is actually longer than the cell itself and roughly 50 times longer than researchers had previously assumed.2PubMed Central. Cell wall peptidoglycan architecture in Bacillus subtilis High-resolution imaging has revealed that these strands form a dynamic architecture that remodels as the cell grows and divides.3PubMed Central. Atomic Force Microscopy of Side Wall and Septa Peptidoglycan From Bacillus subtilis Reveals an Architectural Remodeling During Growth
But peptidoglycan alone does not explain everything about the Gram-positive wall. Threaded through it are teichoic acids, polymers unique to Gram-positive bacteria. B. subtilis has two types: wall teichoic acids (WTA), which are anchored into the peptidoglycan, and lipoteichoic acids (LTA), which are tethered to the cell membrane beneath. These are not optional accessories. Wall teichoic acids are essential for maintaining the rod shape of B. subtilis; when researchers depleted the proteins that build them, cells swelled into spheres and their walls thickened abnormally.4PubMed Central. Teichoic acid is an essential polymer in Bacillus subtilis that is functionally distinct from teichuronic acid More recent work showed that WTA fill tiny pores in the peptidoglycan mesh, limiting which enzymes can access the wall and keeping growth orderly rather than chaotic in all directions.5PubMed Central. Wall teichoic acids plug nanopores in Bacillus subtilis cell wall and regulate peptidoglycan synthesis
Meanwhile, lipoteichoic acids handle a different job. They manage the balance of metal ions like magnesium and calcium near the cell membrane and play a critical role in cell division. Lose LTA and the cell struggles to divide properly; lose both LTA and WTA and the cell simply dies.6PubMed Central. Distinct and essential morphogenic functions for wall- and lipo-teichoic acids in Bacillus subtilis The take-home is that the Gram-positive cell wall is not just a passive barrier that happens to trap dye. It is a structurally sophisticated, actively regulated system that shapes everything about how the organism grows.
When Gram-Positive Bacteria Stain Gram-Negative
Here is where things get less tidy. Older microbiology students and lab technicians know from experience that B. subtilis cultures past their prime often stain pink instead of purple, looking for all the world like Gram-negative organisms. This is not a fluke or a staining error.
A detailed study of Gram variability in bacteria from the Bacillus–Clostridium group found that these organisms stain reliably Gram-positive during lag phase and early growth, but as the culture ages and the doubling time slows, the peptidoglycan wall beneath the surface layer becomes noticeably thinner and more diffuse. By stationary phase, these cultures were described as “virtually gram negative” in their staining behavior.7PubMed Central. Mechanism of gram variability in select bacteria The cells have not changed their fundamental biology or grown an outer membrane. They have simply degraded enough of their peptidoglycan that the dye-trapping mechanism fails.
This matters in practical settings. If you are identifying an unknown bacterial isolate by Gram staining and you use an old or stressed culture, you could easily misclassify B. subtilis as Gram-negative. Good laboratory practice calls for staining cells from young, actively growing cultures to avoid this trap. The Gram stain captures a snapshot of a cell’s wall thickness at a particular moment, not an immutable property of the species.
Sporulation Adds Another Layer of Complexity
B. subtilis is a spore-forming bacterium, and sporulation complicates the staining picture further. When nutrients run out or conditions deteriorate, the cell divides asymmetrically to produce a dormant endospore wrapped in a multilayered protein coat with at least four distinct structural layers.8PubMed Central. The Bacillus subtilis endospore: assembly and functions of the multilayered coat This coat is so chemically resistant that it does not take up crystal violet or safranin in a standard Gram stain. A mature spore in a Gram-stained preparation typically appears as a clear or unstained body, sometimes surrounded by pink-staining remnants of the mother cell that released it.
The spore peptidoglycan itself is structurally different from the vegetative cell wall. It has a much lower degree of cross-linking between peptide side chains, a feature controlled by specific carboxypeptidase enzymes. When researchers knocked out two of these enzymes, the resulting spore peptidoglycan became highly cross-linked, which impaired normal spore dehydration and reduced heat resistance.9PubMed Central. Roles of low-molecular-weight penicillin-binding proteins in Bacillus subtilis spore peptidoglycan synthesis and spore properties This specialized spore wall is distinct enough from the vegetative wall that it further blurs any assumption that Gram-staining behavior is constant across the life cycle.
Assembly of the spore coat itself is a precisely choreographed process. Recent nanoscale imaging showed that inner coat proteins first appear as a ring near the middle of the developing spore and then spread outward in both directions, with later-produced proteins displacing earlier ones toward the poles.10Communications Biology. Real-time nanoscale investigation of spore coat assembly in Bacillus subtilis None of this spore architecture is captured by a simple Gram stain, but it is central to what makes B. subtilis such a resilient organism.
The Premier Model for Gram-Positive Biology
If you have heard of Escherichia coli as the workhorse of Gram-negative research, B. subtilis occupies the equivalent seat on the Gram-positive side. Its genome was sequenced specifically to fill the gap in the list of model organisms for Gram-positive bacteria, and it was chosen because its biochemistry, physiology, and genetics had already been studied intensely for decades.11Nature. The complete genome sequence of the Gram-positive bacterium Bacillus subtilis It remains the best-studied model in the Gram-positive lineage, partly because it is naturally competent, meaning it readily takes up foreign DNA from its environment, making genetic manipulation straightforward.12PubMed Central. Microbe Profile: Bacillus subtilis: model organism for cellular development, and industrial workhorse
Despite their evolutionary distance, B. subtilis and E. coli share some surprising commonalities. Both follow the same general principle of cell-size homeostasis during growth, adding a roughly constant amount of size between divisions regardless of their birth size. Yet at the genetic, molecular, and regulatory level, the two organisms are fundamentally different, which is exactly why studying both has been so productive for understanding the range of bacterial solutions to the same biological problems.13bioRxiv. Gram-positive and Gram-negative Bacteria Share Common Principles to Coordinate Growth and the Cell Cycle at the Single-cell Level
Why Gram Status Matters for Protein Secretion
One of the most practically important consequences of being Gram-positive is how B. subtilis exports proteins. Gram-negative bacteria have to shuttle proteins across two membranes and through the periplasmic space between them. Gram-positive bacteria have only one membrane to cross, after which proteins pass directly through the peptidoglycan wall into the surrounding environment. This architectural simplicity makes B. subtilis an exceptionally efficient protein secretor.
Genome-wide surveys have identified roughly 300 proteins with the potential to be exported from the B. subtilis cytoplasm, distributed across four distinct export pathways. The vast majority travel through the main Sec pathway, while smaller numbers use the twin-arginine translocation (Tat) pathway and other specialized routes.14PubMed. Signal peptide-dependent protein transport in Bacillus subtilis: a genome-based survey of the secretome Researchers have also shown that the Tat pathway in B. subtilis can be redirected to secrete proteins normally handled by the Sec route, opening up new strategies for engineering protein production.15PubMed Central. The twin-arginine signal peptide of Bacillus subtilis YwbN can direct either Tat- or Sec-dependent secretion of different cargo proteins: secretion of active subtilisin via the B. subtilis Tat pathway
This secretion efficiency has made B. subtilis and its close relatives industrial powerhouses. Bacillus species produce an estimated 60% of commercially available enzymes, including proteases used in laundry detergents, amylases for food processing, and various specialty enzymes for pharmaceutical manufacturing.16ScienceDirect. Bacillus Subtilis – Section: Ecology and Significance The organism’s adaptable metabolism and highly efficient secretion system have made it a go-to cell factory across agriculture, medicine, and materials science.17PubMed Central. Bacillus subtilis: a universal cell factory for industry, agriculture, biomaterials and medicine
Biofilms and the Extracellular Matrix
Beyond secreting individual proteins, B. subtilis builds elaborate biofilm communities held together by an extracellular matrix. This matrix is composed predominantly of two main components: a protein called TasA and an exopolysaccharide (EPS). Remove either one and you get a thinner, weaker biofilm; remove both and the organism completely fails to form complex multicellular structures.18PubMed. A major protein component of the Bacillus subtilis biofilm matrix Identifying the first enzymatic steps in EPS synthesis has been a recent breakthrough, giving researchers potential targets to either promote or disrupt biofilm formation on demand.19PubMed Central. Defining early steps in Bacillus subtilis biofilm biosynthesis
The biofilm matrix also serves a defensive function. A third matrix component, the hydrophobin-like protein BslA, coats the colony surface and, together with TasA and EPS, protects B. subtilis from invasion by competitors like Pseudomonas. In plant root environments, this protective matrix determines whether B. subtilis can successfully co-colonize alongside other soil microbes.20PubMed Central. The extracellular matrix protects Bacillus subtilis colonies from Pseudomonas invasion and modulates plant co-colonization The biofilm lifestyle is part of what makes B. subtilis so successful in soil and on plant surfaces, environments where it faces constant competition.
The Gram-Positive/Gram-Negative Divide May Be Blurrier Than You Think
For most of microbiology’s history, the Gram stain was treated as a fundamental dividing line between two kinds of bacteria, roughly mapping onto those with one membrane (monoderms, the Gram-positives) and those with two membranes (diderms, the Gram-negatives). B. subtilis fits neatly on the monoderm side. But evolutionary analysis has made this binary less clean.
Some members of the Firmicutes phylum, the broader group to which B. subtilis belongs, actually possess an outer membrane, making them diderms despite being classified as Firmicutes. Phylogenomic analysis supports the idea that the diderm (two-membrane) architecture is actually the ancestral state in Firmicutes, and that the monoderm phenotype characteristic of B. subtilis and its relatives arose independently multiple times through loss of the outer membrane.21PubMed. One or two membranes? Diderm Firmicutes challenge the Gram-positive/Gram-negative divide A related hypothesis connects this to sporulation: the process of endospore formation involves engulfment of one cell by another, producing a transient double-membrane stage, and mapping cell envelope types onto the tree of life suggests the diderm plan is very ancient.22PubMed Central. Sporulation, bacterial cell envelopes and the origin of life
None of this changes the practical classification of B. subtilis as Gram-positive. But it does mean that the Gram stain captures a phenotype, the thickness and architecture of the cell wall at a given moment, rather than a deep evolutionary identity. Two bacteria can stain the same color for very different evolutionary reasons, and close relatives can sometimes stain differently.
Identifying B. subtilis Beyond the Gram Stain
Because the Gram stain can give ambiguous results depending on culture age and conditions, modern identification of B. subtilis rarely relies on it alone. One increasingly popular method is MALDI-TOF mass spectrometry, which generates a protein fingerprint from a bacterial colony in minutes. Studies have shown it to be more powerful than 16S rRNA gene sequencing for accurately classifying Bacillus species, particularly for distinguishing B. subtilis from closely related species that look nearly identical under a microscope and can behave similarly in biochemical tests.23Food Microbiology. Characterisation and profiling of Bacillus subtilis, Bacillus cereus and Bacillus licheniformis by MALDI-TOF mass fingerprinting The technology has been applied in settings ranging from clinical labs to agricultural research, where identifying specific B. subtilis strains with biocontrol properties matters for crop protection.24Plant Science Today. MALDI-TOF MS identification of Bacillus subtilis SOE 7 against red rot pathogen Colletotrichum falcatum in sugarcane
The Gram stain still has its place as a fast, cheap first step. Under ideal conditions with a fresh culture, it tells you immediately that you are looking at a large, rod-shaped, Gram-positive bacillus, which narrows the field substantially. But given the Gram variability issue and the difficulty of telling Bacillus species apart morphologically, confirmatory identification through molecular or proteomic methods has become standard practice.
B. subtilis Spores as Probiotics
The Gram-positive cell wall and the spore coat together help explain a growing area of commercial interest: B. subtilis as a probiotic. Spores survive stomach acid, bile salts, and the mechanical forces of digestion far better than vegetative cells of most bacteria. In a human trial, spores of the probiotic strain B. subtilis DE111 were detected in the small intestine within three hours of ingestion, and vegetative cells, meaning the spores had germinated and begun growing, appeared at the same time point. Spore recovery peaked around six hours after ingestion, with vegetative cell concentrations peaking at about seven hours.25Frontiers in Microbiology. Presence and Germination of the Probiotic Bacillus subtilis DE111® in the Human Small Intestinal Tract: A Randomized, Crossover, Double-Blind, and Placebo-Controlled Study
That said, the transit appears to be temporary. Work in pig gastrointestinal tracts showed that while the majority of supplemented Bacillus spores germinate in the upper gut, only limited outgrowth of the vegetative population occurs, and the bacteria do not permanently colonize the digestive tract.26Journal of Applied Microbiology. Germination and outgrowth of Bacillus subtilis and Bacillus licheniformis spores in the gastrointestinal tract of pigs This pattern of “pass through, germinate briefly, then leave” is consistent with how spore-forming soil bacteria interact with the mammalian gut: they are transients, not permanent residents. For probiotic purposes, this means consistent daily dosing is necessary to maintain any effect, because the bacteria are not setting up a self-sustaining colony.
An Unusual Peptidoglycan Synthesis System
One of the more surprising findings about B. subtilis cell wall biology came from studies of its peptidoglycan-building enzymes. Researchers constructed a mutant strain lacking all four of the high-molecular-weight enzymes thought to be responsible for polymerizing the glycan strands. By all expectations, this strain should not have been able to make a cell wall at all. Instead, it was viable and produced a peptidoglycan wall with only minor structural differences from the normal version.27PubMed Central. Peptidoglycan synthesis in the absence of class A penicillin-binding proteins in Bacillus subtilis This means B. subtilis harbors at least one unknown enzyme capable of performing the glycan-polymerization step, a finding that challenged a long-standing assumption about how cell walls are assembled. The identity of this backup enzyme became a significant question in the field and points to a level of redundancy in cell wall construction that may partly explain why Gram-positive bacteria are so robust.
More broadly, B. subtilis carries 16 penicillin-binding proteins with distinct roles across vegetative growth and sporulation. Some are dedicated to cell division, others regulate the cross-linking of spore peptidoglycan, and still others remain poorly understood.28FEMS Microbiology Reviews. The penicillin-binding proteins: structure and role in peptidoglycan biosynthesis – Section: Classification and overview of the content in PBPs of selected bacteria This diversity of wall-building machinery underscores that the thick Gram-positive wall is not a static structure but a dynamic, heavily engineered system tuned to different life stages.