Corynebacterium diphtheriae, the bacterium responsible for diphtheria, has one of the more distinctive shapes in microbiology. Individual cells are irregularly shaped rods, often swollen at one end to produce a club-like profile, and they tend to remain connected after dividing in ways that create striking V-shaped pairs, fan-like clusters, and “Chinese letter” arrangements visible under a standard light microscope. These features are not merely cosmetic curiosities; they have been central to identifying the organism since the late 1800s, and modern imaging has revealed that the cell’s unusual silhouette arises from specific growth patterns in its surface. Understanding the morphology also means understanding an envelope structure that sits somewhere between a classic Gram-positive and a Gram-negative bacterium, with a waxy outer membrane that influences everything from antibiotic resistance to how the organism attaches to human tissue.
The Classic Club Shape and How It Forms
Under the microscope, C. diphtheriae cells are roughly 1–8 micrometers long and 0.3–0.8 micrometers wide. They are Gram-positive rods, but they rarely look like neat cylinders. Instead, many cells are wider at one end than the other, giving them the club or “clavate” appearance that inspired the genus name (koryne is Greek for club). Scanning electron microscopy has shown that this swelling corresponds to elevated circular zones on the cell surface where the diameter increases.1PubMed Central. Growth of the surface of Corynebacterium diphtheriae These zones appear to be sites where new cell wall material is being inserted, so the club shape is essentially a snapshot of asymmetric growth caught in the act.
The cells also contain metachromatic granules, sometimes called Babes-Ernst granules or volutin granules, which are deposits of polyphosphate. When stained with methylene blue or Albert’s stain, these granules take on a contrasting color from the rest of the cell body, producing a beaded look. Early bacteriologists relied heavily on this staining pattern for diagnosis, and it remains a useful feature in resource-limited settings where molecular testing is unavailable. The granules tend to sit near the poles of the cell, reinforcing the visual impression that the ends of the rod are somehow different from the middle.
Why the Cells Arrange Themselves in V-Shapes and Palisades
One of the most recognizable features of C. diphtheriae on a stained slide is the way cells stay attached to each other after dividing. Rather than separating cleanly and drifting apart, daughter cells often remain hinged at the point where they split, producing angular V-shaped or L-shaped pairs. When several generations of cells do this in a row, the result is a cluster that looks like letters of the Chinese alphabet, a comparison microbiologists have used for well over a century. Cells can also line up side by side in neat rows called palisades, resembling a picket fence viewed from above.
This distinctive post-division behavior comes from the way C. diphtheriae builds its septum, the internal wall that divides one cell into two. Electron microscopy has shown that the cell wall and cytoplasmic membrane grow inward together to form this dividing wall.2Japanese Journal of Microbiology. Electron Microscopy of the Fine Structure of Corynebacterium diphtheriae, with Special Reference to the Intracytoplasmic Membrane System The septum doesn’t break apart evenly on all sides at once. Instead, it snaps open on one side first, causing the daughter cells to swing apart like a book opening, which is what produces the V-angle. This snapping division, sometimes called “post-fission snapping,” is characteristic of corynebacteria generally and is a major reason they look so different from other rod-shaped bacteria under the microscope.
When the gene for a protein called DIP1281, which helps organize the cell surface, is knocked out, this orderly pattern breaks down. Mutant cells grow in chains rather than V-shapes or palisades, and they also become larger and less club-like in shape, demonstrating that maintaining the classic morphology requires active coordination by surface-organizing proteins.3PubMed Central. Corynebacterium diphtheriae invasion-associated protein (DIP1281) is involved in cell surface organization, adhesion and internalization in epithelial cells
The Unusual Cell Envelope
C. diphtheriae stains Gram-positive, which normally implies a thick peptidoglycan wall and no outer membrane. But the reality is more complicated. Like its relatives in the order Mycobacteriales, including the tuberculosis bacterium, C. diphtheriae has an additional outer membrane called the mycomembrane (sometimes referred to as the mycolic acid layer). This structure is built from long-chain fatty acids called mycolic acids, and it wraps around the outside of the cell wall to create a waxy, hydrophobic barrier.4Current Biology. The Mycobacteriales cell envelope The mycolic acids in C. diphtheriae are shorter than those in Mycobacterium tuberculosis, which is one reason corynebacteria are easier to stain and generally less resistant to chemical attack, but the basic architecture is shared.
Beneath the mycomembrane sits a two-layered cell wall made of peptidoglycan and arabinogalactan, and beneath that sits the cytoplasmic membrane. Early transmission electron microscopy described the cell wall as having three layers: dense outer and inner layers separated by a less-dense intermediate layer.5Japanese Journal of Microbiology. Electron Microscopy of the Fine Structure of Corynebacterium diphtheriae, with Special Reference to the Intracytoplasmic Membrane System More recent cryo-electron microscopy has measured the mycomembrane itself at about 7 nanometers thick, and shown that it consists of two distinct leaflets that can be resolved in cross-section.6PLoS Biology. Mapping the ultrastructural topology of the corynebacterial cell surface
Some strains of C. diphtheriae also have an S-layer, a crystalline protein coat that sits on top of the mycomembrane. When the S-layer is present, the two leaflets of the mycomembrane underneath appear interrupted and discontinuous under high-resolution imaging, possibly because the S-layer’s anchoring segments physically insert into and disrupt the membrane.7PLoS Biology. Mapping the ultrastructural topology of the corynebacterial cell surface When there is no S-layer, the mycomembrane looks smooth and uninterrupted. This interaction between surface coat and outer membrane is an active area of research because it could affect how the bacterium resists immune attack and takes up nutrients.
How the Mycomembrane Matters Practically
The mycomembrane functions as a selective permeability barrier, much like the outer membrane of Gram-negative bacteria. It blocks large and water-loving molecules from entering the cell, which includes many antibiotics.8Current Biology. The Mycobacteriales cell envelope Some studies have even suggested that the barrier function of the mycomembrane can exceed that of the Gram-negative outer membrane, owing to the density and thickness of its lipid composition. For a bacterium that colonizes the human throat and skin, this waxy shield also helps it survive environmental stresses like drying, temperature shifts, and the chemical defenses of mucous membranes.
To allow essential nutrients through this barrier, C. diphtheriae has channel-forming proteins embedded in its cell wall. One such protein, with a molecular mass of about 66 kilodaltons, was found to form pores that permit the passage of small molecules across the outer layers of the envelope.9Journal of Bacteriology. Corynebacterium diphtheriae: identification and characterization of a channel-forming protein in the cell wall These porins are analogous in function to the outer membrane porins of Gram-negative bacteria, which is another example of how C. diphtheriae’s envelope straddles the traditional Gram-positive/Gram-negative divide. This hybrid architecture is one of the reasons some researchers describe the Mycobacteriales as “Gram-variable” or simply as a category unto themselves.
Pili on the Surface
C. diphtheriae assembles at least three distinct types of pili on its surface. These are hair-like protein filaments that project outward from the cell and play a critical role in attaching to human tissue. The best-studied pilus is built from a major structural protein called SpaA, which forms the shaft, along with two minor subunits, SpaB and SpaC.10PubMed Central. Assembly of distinct pilus structures on the surface of Corynebacterium diphtheriae Assembly of these pili depends on enzymes called sortases that covalently link pilin subunits together in a chain, a process where each subunit is essentially clicked into the next through a chemical bond between specific amino acid residues.11PubMed Central. Sortase-assembled pili in Corynebacterium diphtheriae are built using a latch mechanism
The three pilus types are not equally important for all kinds of attachment. In experiments with human pharyngeal epithelial cells, deleting the gene for SpaA abolished the pilus structure itself but did not eliminate the bacterium’s ability to adhere to those cells, suggesting that the minor pilin subunits or other surface molecules can compensate for the loss of the main shaft.12PubMed Central. Corynebacterium diphtheriae employs specific minor pilins to target human pharyngeal epithelial cells This redundancy makes sense for a pathogen: relying on a single adhesion mechanism would make it vulnerable to any host defense or mutation that blocked that one pathway.
From a morphological standpoint, pili are too thin to see under a standard light microscope, but they show up clearly in electron micrographs as fine filaments radiating from the cell surface. Their presence adds texture and functional complexity to what already looks like a busy exterior when viewed at high magnification.
How Antibiotics and Growth Conditions Change the Shape
The classic morphology described above is what you see under standard laboratory growth conditions. Change those conditions, and C. diphtheriae can look quite different. Exposure to sub-inhibitory concentrations of penicillin, levels too low to kill the bacteria but high enough to interfere with cell wall synthesis, causes the cells to form long filaments rather than dividing normally. Meanwhile, low-level exposure to erythromycin, which targets protein synthesis rather than the cell wall, causes cells to shrink in size.13PubMed. SubMICs of penicillin and erythromycin enhance biofilm formation and hydrophobicity of Corynebacterium diphtheriae strains Both of these changes are accompanied by increased biofilm formation and greater cell-surface hydrophobicity, which could make the bacteria harder to clear from infected tissue even though the antibiotic isn’t at full killing strength.
This is a meaningful clinical concern. Antibiotic concentrations in the throat or on the skin are not uniform: at the edges of a treated area or between doses, bacteria may encounter sub-inhibitory drug levels. If those concentrations push C. diphtheriae into a filamentous, biofilm-forming state rather than simply killing it, treatment could paradoxically make the infection more tenacious in certain niches.
Biofilm Formation and Its Effect on Cells
Like many bacteria, C. diphtheriae can transition from free-floating individual cells to communities embedded in a self-produced matrix. In this biofilm state, the bacteria can change their properties: cell sizes shift, antibiotic sensitivity decreases, and the balance between sticking to surfaces and invading deeper tissues is altered. Specifically, when biofilm forms, adhesion increases while invasiveness drops sharply.14Epidemiology and Vaccinal Prevention. Corynebacterium diphtheria, Adhesion, Type and Biofilm Cultures The bacterium essentially trades its ability to penetrate tissue for a stronger grip on the surface and a communal shield against threats.
Under the microscope, biofilm-associated C. diphtheriae cells often look different from their planktonic counterparts. They may appear more uniform in size, pack more tightly together, and produce an extracellular matrix visible as a hazy background in stained preparations. The shift from classic V-shaped pairs to dense, mat-like communities is one reason laboratory identification sometimes fails when working with biofilm-derived samples: the cells simply don’t look the way the textbook says they should.
Fibrinogen Binding and Clumping
An aspect of C. diphtheriae morphology that matters directly for disease is the bacterium’s tendency to clump together in the presence of blood plasma. When exposed to plasma, both toxigenic and nontoxigenic strains form visible aggregates, indicating the presence of a clumping factor on the cell surface that interacts with fibrinogen.15Memórias do Instituto Oswaldo Cruz. Fibrinogen binds to nontoxigenic and toxigenic Corynebacterium diphtheriae strains Toxigenic strains, the ones carrying the gene for diphtheria toxin, show higher affinity for plasma-mediated aggregation than nontoxigenic strains. This clumping behavior is directly relevant to how the classic diphtheritic pseudomembrane forms in the throat: bacteria, fibrin, dead cells, and inflammatory debris weave together into a tough, adherent mat that can obstruct the airway. The morphology of the bacterium in tissue, then, looks nothing like neat V-shaped pairs; it is tangled masses embedded in fibrin.
What Staining Reveals and What It Misses
In routine clinical microbiology, C. diphtheriae is identified in part by its appearance on Gram stain and special stains like Albert’s stain or Loeffler’s methylene blue. On Gram stain, the cells appear as irregular, often beaded Gram-positive rods in angular or palisade arrangements. The metachromatic granules show best with methylene blue, where they stain a deeper blue-black against the lighter blue cell body. Albert’s stain uses toluidine blue and malachite green to make the granules stand out in green against a blue-green cell body, or in some protocols, the reverse.
These staining methods work well under textbook conditions, but there are real-world limitations. Overgrowth by normal throat flora can obscure the characteristic arrangements. Old cultures may lose the club shape and the granules. And as noted above, bacteria from biofilms or patients partially treated with antibiotics may not display the classic morphology at all. This is why modern guidelines do not recommend relying solely on microscopy for diphtheria diagnosis; culture on selective media like potassium tellurite agar, followed by biochemical testing and toxigenicity assays, remains the standard. Still, for a trained microscopist, the morphology provides a fast and free preliminary clue that can speed up the diagnostic process by hours.
How C. diphtheriae Compares to Other Corynebacteria
The genus Corynebacterium includes dozens of species, many of which are harmless residents of human skin and mucous membranes. They all share the basic corynebacterial body plan: irregular rods with a tendency toward club shapes and angular post-division arrangements. What distinguishes C. diphtheriae morphologically from its non-pathogenic relatives is partly a matter of degree. The metachromatic granules tend to be more prominent in C. diphtheriae than in many commensal species, and the V-shaped and palisade arrangements are usually more regular and well-defined. But these differences are not absolute, and misidentification based on morphology alone is a real risk, especially with species like C. ulcerans and C. pseudotuberculosis, which are close relatives that can also carry diphtheria toxin genes.
At the ultrastructural level, the cell envelope architecture is broadly conserved across the Mycobacteriales, with the mycomembrane being a shared feature. The industrial workhorse C. glutamicum, widely used in amino acid production, has a very similar envelope layout and has been used extensively as a model organism for studying corynebacterial cell wall biology. Findings from C. glutamicum studies often translate to C. diphtheriae, though the details of pilus structure, S-layer presence, and surface protein repertoire differ between species.
Iron Limitation and Surface Remodeling
Inside the human body, iron is scarce because the host actively sequesters it as a defense against infection. C. diphtheriae responds to iron limitation by extensively remodeling its surface proteins. This remodeling has significant morphological consequences at the molecular level: the bacterium upregulates iron-scavenging systems on its exterior, changes the composition of its outer surface proteome, and shifts the balance of proteins it secretes into the surrounding environment. While these changes are not visible under a standard light microscope, they alter the functional surface of the cell in ways that affect how it interacts with host tissue and immune cells. The toxin gene itself is famously regulated by iron: diphtheria toxin is produced most abundantly when iron is scarce, linking the bacterium’s metabolic state directly to its virulence. So the same environmental condition that triggers surface remodeling also turns on the organism’s most dangerous weapon.
For researchers imaging C. diphtheriae from clinical specimens versus laboratory cultures, this means the surface they see depends heavily on growth conditions. A bacterium pulled from an iron-starved environment in the human throat presents a different molecular face than the same strain grown in rich laboratory broth. This contextual variability is an important reminder that morphology in microbiology is never entirely static: it is a product of genetics, environment, and the specific moment of observation.