Bacillus subtilis colonies on agar are strikingly variable. Depending on the strain, the nutrient level, and the agar concentration, you might see anything from a flat, smooth, off-white disc to a dramatically wrinkled, ridged structure that looks almost like a dried-out riverbed. The organism is one of the most-studied bacteria on the planet, yet even experienced microbiologists can be caught off guard by how different two plates of the same species can look. That variability is not a flaw in your technique; it is a defining feature of the organism and one of the reasons it remains so useful as a research model.
The Classic Colony Appearance
When you streak B. subtilis on a standard nutrient agar plate and incubate it overnight at around 37 °C, the colonies that appear are typically dry-looking, irregularly edged, and off-white to beige. Unlike the smooth, glossy, dome-shaped colonies you might see from something like E. coli, B. subtilis colonies tend to have a rough, matte surface and margins that look wavy or lobate rather than neatly circular. The texture is often described as “fried-egg” or “ground-glass,” especially on richer media, and the colonies can feel almost papery if you try to pick them up with a loop. They do not glisten with moisture the way many Gram-negative colonies do.
That said, this textbook description applies mostly to commonly used lab strains growing on general-purpose media. The reality in any given experiment is far more interesting, because B. subtilis can rearrange its colony architecture dramatically in response to changes in its environment.
How Nutrients and Agar Concentration Reshape the Colony
One of the most striking things about B. subtilis is how many different growth patterns a single strain can produce simply by adjusting what is in the plate. A classic study catalogued the range: the same strain formed fractal-looking colonies on hard, nutrient-poor agar; round, compact colonies on richer plates; densely branched structures on intermediate media; colonies that spread into a continuous film without gaps; and colonies with concentric rings. The microstructure within each of these patterns was distinct, and the shifts were linked to how easily nutrients could diffuse through the gel.1FEMS Microbiology Ecology. Diversity of the growth patterns of Bacillus subtilis colonies on agar plates
In practical terms, this means that if you pour your plates a little thicker than usual, or if you use a richer broth base, the colony you get at 48 hours can look completely different from what you expected. Harder agar (higher concentration) tends to restrict spreading and push colonies toward compact, sometimes fractal shapes. Softer agar allows more motility and spreading, producing thinner, flatter colonies that can cover a large area. Nutrient concentration matters just as much: starving the bacteria tends to promote expansion outward, while rich media can encourage denser, more compact growth.
Wrinkles, Folds, and Biofilm Architecture
If you let a B. subtilis colony grow for more than a day or two on biofilm-promoting media, the surface develops conspicuous wrinkles. These are not random. The folds follow reproducible patterns that change over time and respond to nutritional conditions. On nutrient-depleted media, the wrinkles tend to be large and straight, running from the center of the colony out to the edge like spokes on a wheel. On richer media, the folds are more disorganized and labyrinthine.2PubMed Central. The Adaptive Morphology of Bacillus subtilis Biofilms: A Defense Mechanism against Bacterial Starvation
A detailed time-course study tracked wrinkle evolution in colony biofilms over more than 20 days and identified five successive stages. The center of the colony first develops chaotic concentric rings, which give way to labyrinthine networks, then radial ridges, then branching structures, and finally complex “burgeons” where individual wrinkles split into three or more sub-branches. As the patterns evolve outward, the wrinkles get thinner and closer together, but the overall density increases.3Journal of Mechanics in Medicine and Biology. The Evolving Wrinkle Pattern of the Bacillus Subtilis Biofilm Providing More Living Space for Cells
These wrinkles are not just decorative. They are structural features of the biofilm that help channel fluids, separate cell populations, and create more surface area for a growing community. A protein called BslA coats the outer surface of the biofilm and makes it hydrophobic, giving mature colonies that distinctive dry, water-repellent feel. When the gene for BslA is knocked out, the surface loses its repellency and the fine microstructure of the biofilm changes.4PubMed. BslA(YuaB) forms a hydrophobic layer on the surface of Bacillus subtilis biofilms
Wild Strains Versus Lab Strains
If your plates look disappointingly flat and featureless, there is a good chance you are working with a domesticated lab strain. The most widely used B. subtilis strain, called 168, has been passed through laboratories for decades and has accumulated mutations that can weaken its ability to form robust biofilms. Compared with the undomesticated wild strain NCIB 3610, strain 168 generally produces thinner, less architecturally complex colonies, even when researchers have tried to repair specific known mutations.5PubMed Central. Tracing the domestication of a biofilm-forming bacterium
The story is more complicated than “168 is bad at biofilms,” though. A survey of numerous 168 stocks from different labs found wide variation among them. Some were indeed unable to build robust biofilm structures, but others formed colonies just as architecturally complex as the wild NCIB 3610 strain. The differences came down to which specific sub-lineage of 168 a given lab happened to maintain.6PubMed. Specific Bacillus subtilis 168 variants form biofilms on nutrient-rich medium So if you are comparing your colonies to a colleague’s and getting very different results, the source of your strain matters as much as the media recipe.
Color Changes and the Red Pigment
Fresh B. subtilis colonies are typically off-white to cream-colored. But under certain conditions, you can see unexpected color shifts that tell you something about what the bacteria are doing.
The most eye-catching is a reddish pigment called pulcherrimin. On iron-supplemented media, wild-type B. subtilis colonies develop a visible reddish halo around them. The bacterium secretes a precursor molecule called pulcherriminic acid, which grabs free iron from the medium in a chemical reaction that produces the insoluble red compound. The more iron available, the more intense the color.7PubMed Central. Pulcherrimin protects Bacillus subtilis against oxidative stress during biofilm development This is not just a cosmetic curiosity. Pulcherrimin production is directly tied to biofilm growth arrest: when the bacteria have scavenged enough iron from their surroundings to drop available iron below a critical threshold, the colony stops expanding.8PubMed Central. Pulcherrimin formation controls growth arrest of the Bacillus subtilis biofilm
Aside from pulcherrimin, colony color shifts over time as cells sporulate. Young colonies are pale and somewhat translucent. As sporulation ramps up, typically after two or three days, colonies darken to an opaque brown or tan. The brown color comes from the spore coat pigments and the sheer density of refractile spores packed into the colony. If you are working with a strain that has lost its ability to sporulate, its colonies stay whiter and become increasingly translucent the longer they incubate, which is a quick visual clue that something has changed genetically.9PubMed Central. Phosphorelay changes and plasticity underlie the life history evolution of Bacillus subtilis sporulation and germination in serial batch culture
Swarming and Dendritic Spreading
On softer agar, B. subtilis does not just grow outward passively. It actively swarms, sending waves of motile cells across the surface. Wild-type strain 3610 on a synthetic medium forms highly branched, tree-like (dendritic) swarming patterns generated by successive pulses of moving cells.10PubMed. Branched swarming patterns on a synthetic medium formed by wild-type Bacillus subtilis strain 3610 The result looks less like a traditional bacterial colony and more like frost spreading across a windowpane: thin tendrils extending outward with secondary branches splitting off at irregular intervals.
Swarming and biofilm formation are driven by different cell types within the B. subtilis population. Cells that are flagellated and motile drive the swarming front, while cells producing extracellular matrix components stay behind and build up the biofilm core. On a plate where swarming is possible, you often see a thin leading edge of nearly transparent growth surrounding a thicker, textured central mass. The interplay between these two behaviors is one reason why colony morphology is so sensitive to agar hardness: a small change in gel firmness can shift the balance between swarming and sessile biofilm growth.
What the Inside of a Colony Looks Like
Viewed from above, a B. subtilis biofilm colony can appear uniform, but the interior is anything but. Fluorescent reporter studies have shown that the colony is organized into concentric zones of different cell types. Matrix-producing cells, the ones responsible for the structural glue holding the biofilm together, are concentrated in a narrow ring near the expanding edge, within roughly two millimeters of the leading front. Sporulating cells accumulate farther from the edge, in the older interior of the colony. Underneath the sporulating zone, a thin layer of residual matrix-producing cells persists near the agar surface.11Biophysical Journal. Spatiotemporal Dynamics of Proliferating Bacillus subtilis Biofilms
Sporulation itself spreads through the colony in a wave. It initiates near the original inoculation point and moves bidirectionally: toward the center and toward the actively expanding leading edge. As cells complete spore development, they go dark in fluorescent imaging, reflecting full dormancy.12bioRxiv. Colony morphogenesis regulates sporulation dynamics in bacterial biofilms What this means visually is that the central part of an older colony is packed with dormant spores (contributing to its opaque, brownish look), while the actively growing edge remains lighter and more translucent.
Mass spectrometry imaging has added another layer to the picture. Researchers have mapped the spatial distribution of specific molecules across the colony surface and found that certain cannibalistic toxins and signaling peptides are concentrated in particular zones, correlating with changes in biofilm morphology when key regulatory genes are deleted.13PubMed Central. Characterization of Bacillus subtilis Colony Biofilms via Mass Spectrometry and Fluorescence Imaging So the wrinkles and rings you see on the surface are, in a real sense, visible signatures of a molecularly zoned community.
Behavior on Diagnostic and Specialty Media
In teaching and clinical labs, B. subtilis is often grown on specialty media that reveal specific enzymatic activities. On starch agar, the colonies produce amylase, an enzyme that breaks down starch. When you flood the plate with iodine solution after incubation, undigested starch turns purple, but a clear halo appears around B. subtilis colonies where the starch has been hydrolyzed. The size of the clear zone gives a rough indication of how much amylase the strain produces.14PubMed Central. Optimization of Alkaline α-Amylase Production by Thermophilic Bacillus subtilis
On blood agar, the picture depends on the strain. Some B. subtilis strains produce surfactin or related lipopeptides that lyse red blood cells, creating clear hemolytic zones around the colonies. Other strains, including the standard 168 lab strain, produce no hemolysis at all. In strains that do hemolyze, the zones grow larger over time, and the ratio of hemolytic zone to colony size can be measured by image analysis. Because B. subtilis colonies are often irregular in shape rather than neatly circular, manual measurement of these zones can be tricky, and automated imaging methods have been developed for consistency.15PubMed Central. High-throughput evaluation of hemolytic activity through precise measurement of colony and hemolytic zone sizes of engineered Bacillus subtilis on blood agar The hemolytic and antifungal activities trace back to genes involved in surfactin and fengycin synthesis; knocking out those pathways eliminates the hemolytic zones entirely.16PubMed. Genetic analysis of the biosynthesis of non-ribosomal peptide- and polyketide-like antibiotics, iron uptake and biofilm formation by Bacillus subtilis A1/3
How Neighbors Change Colony Shape
Growing B. subtilis next to other species on the same plate can visibly alter its colony morphology. When co-cultured with certain Streptomyces species on agar, B. subtilis launches a mobile response, physically migrating away from or toward the neighboring organism in patterns not seen when it grows alone.17PubMed Central. Antibiotic Stimulation of a Bacillus subtilis Migratory Response The response appears to be triggered by antibiotics or other secondary metabolites produced by the competitor.
Even two B. subtilis colonies on the same plate influence each other. Two biofilms inoculated at different distances grow asymmetrically: their shapes elongate or compress depending on whether the neighboring colony is close enough to compete for nutrients or far enough away that chemical signals promote expansion. The result is that colonies near a neighbor become visibly lopsided, expanding more vigorously on the side facing away from the competitor and growing more slowly on the side facing it.18PubMed. Cell differentiation and motion determine the Bacillus subtilis biofilm morphological evolution under the competitive growth If you see asymmetric or oddly shaped colonies on a plate, crowding or proximity to neighbors is often the explanation.
Why Your Plate Might Not Match the Textbook Photo
Given everything above, it is worth listing the most common reasons a B. subtilis plate looks “wrong” compared to expectations. Most of these are perfectly normal variations, not contamination or failed technique.
- Agar too soft: Colonies spread into a thin, nearly transparent film rather than forming discrete raised colonies. This is swarming, not contamination.
- Nutrient-poor media: Colonies expand further, develop prominent radial wrinkles, and look more dramatic than typical textbook images of compact growth on rich agar.
- Domesticated strain: If you are using 168 or a derivative, your biofilm may look flat and featureless compared to published photos of wild-type NCIB 3610 colonies.
- Extended incubation: Colonies darken to brown as spores accumulate, and wrinkle patterns become more elaborate over several days.
- Iron in the media: A reddish halo or tint appears due to pulcherrimin production, easily mistaken for contamination if you are not expecting it.
- Neighboring colonies: Crowded plates produce asymmetric, misshapen colonies that look different from isolated ones.
B. subtilis is one of those organisms where understanding the conditions matters as much as knowing the species. Two researchers can describe completely different-looking plates and both be correct. The diversity of morphologies is not a bug; it is one of the reasons the organism has become a go-to system for studying how bacterial communities organize themselves.
Distinguishing B. subtilis From Relatives on a Plate
In mixed cultures or environmental samples, the visual challenge is telling B. subtilis apart from close relatives like Bacillus cereus or Bacillus licheniformis. All three form large, irregular, dry-looking colonies on nutrient agar, and colony morphology alone is generally not reliable enough for species-level identification. B. cereus colonies tend to be slightly waxier and more opaque, sometimes with a ground-glass texture and a more regular margin, but the overlap is considerable. B. licheniformis can produce mucoid colonies that are more glistening than typical B. subtilis, especially on fresh isolation, though older colonies dry out and look similar.
A starch hydrolysis test helps narrow things down, since B. subtilis is a strong amylase producer, but so is B. licheniformis. Hemolysis patterns offer another clue: B. cereus is typically strongly hemolytic, while B. subtilis varies by strain. In practice, most labs confirm identity with molecular methods or mass spectrometry fingerprinting rather than relying solely on plate morphology, because the visual overlap among Bacillus species is just too large. The colony appearance is a useful first screen, not a definitive answer, and the range of normal variation within B. subtilis itself makes visual identification even trickier.