Pleomorphism is the ability of a living cell or virus to change its shape, sometimes dramatically, in response to its environment. Bacteria can morph from rods into spheres, filaments, or even wall-less blobs; fungi can toggle between yeast and thread-like forms; viruses can assemble as tiny spheres or as filaments dozens of times longer; and cancer cells can display wildly irregular sizes and shapes under a microscope. Far from being a curiosity, this shape-shifting has real consequences for how infections resist treatment, how tumors progress, and how diagnoses go wrong.
How Bacteria Control Their Shape
A bacterium’s shape is not an accident. It is actively maintained by internal scaffolding, much like the structural beams in a building. The most studied piece of that scaffolding is a protein called MreB, which forms filament bundles just beneath the cell membrane. MreB guides where new wall material gets inserted during growth, and its arrangement determines whether a cell ends up rod-shaped or spherical. Modeling work has shown that cells growing without MreB become unstable and round up, but the transformation is reversible: restore MreB activity, and rod shape returns.1PubMed Central. Mechanical control of bacterial cell shape A related protein, FtsZ, cooperates with MreB but handles different jobs. In the stalked bacterium Caulobacter, for example, MreB is needed for lengthening a thin appendage called the stalk, while FtsZ drives a burst of wall synthesis that builds internal cross-band structures.2PubMed. The cell shape proteins MreB and MreC control cell morphogenesis by positioning cell wall synthetic complexes
The interplay between MreB and FtsZ illustrates how evolution has built flexibility into bacterial architecture. When environmental conditions change, cells can shift the balance between these cytoskeletal systems, altering their shape without needing to mutate. That built-in flexibility is a major reason bacteria can adapt so quickly to hostile environments.3PubMed Central. MreB: unraveling the molecular mechanisms of bacterial shape, division, and environmental adaptation
L-Form Bacteria and Life Without a Wall
The most extreme version of bacterial pleomorphism involves ditching the cell wall entirely. Under certain stresses, bacteria can shed their rigid outer layer and become so-called L-forms: soft, irregularly shaped cells that look nothing like their walled parents. These L-forms can still grow and divide, but they do it by a fundamentally different mechanism. Instead of the precise binary fission that walled bacteria use, L-forms proliferate through an imbalance between membrane growth and cell volume. When the membrane grows faster than the interior, the cell bulges, deforms, and eventually pinches apart.4PubMed Central. L-form bacteria, chronic diseases and the origins of life
The transition is reversible. Once the stress that triggered the switch is removed, L-form bacteria can rebuild their walls and revert to their original shape.5PubMed Central. Bacterial L-forms: Key Mechanisms of Drug Resistance, Disease Recurrence, and Immune Evasion This reversibility is what makes L-forms medically worrying. A bacterium that drops its wall during antibiotic treatment might survive in its L-form state, then regrow the wall and cause a relapse after the drug course ends.
Sustaining L-form life is not simple, though. Research on Bacillus subtilis has found that two classes of genetic changes are needed for L-forms to thrive. One class ramps up membrane production to fuel that membrane-driven division. The other class deals with a metabolic side effect: wall-less cells experience a surge of reactive oxygen species from their respiratory chain, and without mutations that dial down that oxidative stress, L-forms struggle to survive.6PubMed Central. L-form bacteria, chronic diseases and the origins of life
Shape-Shifting as Antibiotic Resistance
Bacteria do not just change shape randomly under antibiotic pressure. The direction of the shape change tracks with how the antibiotic works. Modeling and experimental data show that when a drug enters the cell through the outer surface, bacteria reduce their surface-to-volume ratio, essentially becoming rounder and fatter, which decreases the membrane area available for the drug to cross and lowers the concentration of antibiotic inside the cell. Conversely, when a drug targets the membrane itself, bacteria increase their surface-to-volume ratio, stretching out or forming filaments. The added membrane area dilutes the drug molecules sitting in it, reducing the effective dose per unit of membrane.7PubMed Central. Antibiotic Resistance via Bacterial Cell Shape-Shifting
The L-form switch is another version of this strategy. Under antibiotic pressure, bacteria may shed their cell walls and enter the L-form state specifically to evade drugs that target wall synthesis. Research on both Gram-negative and Gram-positive species has demonstrated that cells can be induced into L-forms by combinations of lysozyme and antibiotics like meropenem.8PubMed. Response mechanisms of resistance in L-form bacteria to different target antibiotics: Implications from oxidative stress to metabolism In Mycobacterium tuberculosis, the bacterium behind tuberculosis, L-form transformation has been linked to tolerance of the drug ethambutol. Because ethambutol works by blocking cell-wall construction, a bacterium without a wall simply has no target for the drug to hit. L-form TB bacteria may also produce capsule-like structures and biofilm that further shield them.9PubMed. L-form transformation phenomenon in Mycobacterium tuberculosis associated with drug tolerance to ethambutol
Environmental stress beyond antibiotics also triggers morphological shifts. Bacterial filamentation, where a cell elongates into a long chain without dividing, is one of the most common stress responses. Research on E. coli has shown that filamentation can be part of an adaptive strategy linked to metabolic trade-offs: filament formation and the ability to use alternative carbon sources like citrate are connected through a molecular signaling circuit, revealing hidden links between a bacterium’s shape and its metabolism.10ASM Journals / Microbiology Spectrum. Metabolic and Morphotypic Trade-Offs within the Eco-Evolutionary Dynamics of Escherichia coli
Evading the Immune System Through Shape
Shape-shifting does more than dodge antibiotics. It can also help bacteria avoid being eaten by immune cells. Macrophages, the immune system’s primary scavengers, engulf pathogens through a process called phagocytosis. But the geometry of the target matters enormously. Work with filamentous E. coli has shown that macrophages can only successfully engulf an elongated bacterium if they first grab one of its two ends. A macrophage that contacts a filament along its middle or side cannot initiate uptake. In a crowded tissue environment where orientation is random, this gives filamentous bacteria a built-in survival advantage: many encounters with immune cells simply fail because the geometry is wrong.11PubMed. The race to the pole: how high-aspect ratio shape and heterogeneous environments limit phagocytosis of filamentous Escherichia coli bacteria by macrophages
This finding has implications beyond infection biology. Bioengineers designing drug-delivery nanoparticles have borrowed the same insight, exploring whether high-aspect-ratio shapes can help therapeutic particles avoid immune clearance just as filamentous bacteria do. The lesson from pleomorphism, that shape and biological fate are tightly linked, cuts across microbiology and nanotechnology alike.
When Bacteria Play Dead
Some pleomorphic transitions go beyond shape changes and involve entering a dormant-like state. Helicobacter pylori, the bacterium that causes stomach ulcers, is a well-studied example. Under hostile conditions such as antibiotic exposure or nutrient depletion, H. pylori converts from its usual spiral or rod form into a coccoid (spherical) form. This transition is not instantaneous. Studies tracking the conversion have documented a progression from U-shaped cells around day seven, to doughnut-shaped cells around day nine, to fully coccoid cells by roughly day fifteen.12PubMed Central. Changes in the evolution of the antigenic profiles and morphology during coccoid conversion of Helicobacter pylori
The coccoid form retains internal cellular structures similar to those of the rod form, but it sheds some surface proteins, including ones the immune system normally recognizes. That makes coccoid H. pylori harder for the body to detect. These spherical cells can enter a viable-but-not-culturable state, meaning they are metabolically active but refuse to grow on standard lab media. Over 85 bacterial species, including dozens of human pathogens, are known to exhibit this behavior, and it helps explain why some infections seem to clear with treatment but come roaring back weeks or months later.13PubMed Central. Coccoid H. pylori: A key driver of recurrent infections and a target for novel therapies
Fungal Shape-Shifting and Virulence
Pleomorphism is not limited to bacteria. Candida albicans, the most common cause of yeast infections, is a textbook example in fungi. It lives harmlessly in the human gut and on mucosal surfaces in its round, budding yeast form. But when conditions change, such as a rise in temperature, a shift in pH, or signals from the host’s own cells, C. albicans switches to a filamentous hyphal form: long, branching threads that can physically penetrate tissue. This morphological switch is one of the organism’s key virulence traits, and it is reversible.14PubMed Central. From Jekyll to Hyde: The Yeast-Hyphal Transition of Candida albicans
The transition is regulated by a range of environmental cues that the fungus encounters in different host niches. Signals like body temperature, serum, carbon dioxide levels, and nutrient availability all feed into a network of signaling pathways that activate or suppress the hyphal growth program.15PubMed Central. Transcriptional control of hyphal morphogenesis in Candida albicans For a long time, a signaling cascade involving the molecule cAMP was considered the central driver. Recent multi-omics studies have complicated that picture, showing that cAMP-independent mechanisms also push morphological changes in C. albicans.16PubMed Central. Shape-Shifting Mechanisms: Integrative Multi-Omics Insights Into Candida albicans Morphogenesis The upshot is that C. albicans has redundant control over its shape-shifting, making it harder to block with a single drug or intervention.
The cAMP signaling pathway turns up in surprising places beyond human pathogens. In nematode-trapping fungi like Arthrobotrys oligospora, the same conserved pathway controls whether the fungus builds the sticky traps it uses to catch and consume tiny roundworms. Deleting the gene for a key enzyme in the pathway left the fungus unable to sense or respond to the presence of its prey, and its trap formation collapsed. Providing cAMP externally rescued the defect.17G3 Genes|Genomes|Genetics. The cAMP-PKA pathway regulates prey sensing and trap morphogenesis in the nematode-trapping fungus Arthrobotrys oligospora It is a striking case of the same molecular toolkit being used for vastly different kinds of shape change across the fungal kingdom.
Viral Pleomorphism
Viruses, though not alive in the traditional sense, also display pleomorphism. Influenza is the best-known example. Clinical isolates of influenza freshly taken from patients produce a mix of particle shapes, including extremely long filamentous virions. But when the same virus strains are grown repeatedly in laboratory cell cultures, they tend to lose filament production and shift almost entirely to spherical particles.18PubMed Central. Filamentous influenza viruses This matters because much of what researchers know about influenza comes from lab-adapted strains, meaning the filamentous forms that dominate real infections have historically been understudied.
The host cell itself plays a role in determining viral shape. When the A/Udorn/72 influenza strain infects polarized epithelial cells, the cell type that lines the airways, it produces filaments up to 30 micrometers long. The same virus strain infecting non-polarized cells produces almost exclusively spherical particles.19PubMed. Host cell dependence of viral morphology The virus’s shape is therefore not entirely encoded in its own genome; it emerges from an interaction between viral proteins and the architecture of the host cell. This host-dependent pleomorphism could influence how easily the virus spreads within the respiratory tract, since filamentous virions may carry more copies of surface proteins and interact differently with mucus and immune cells than spherical ones do.
Pleomorphism in Cancer Cells
In oncology, pleomorphism refers to the abnormal variation in size and shape of tumor cells and their nuclei. Pathologists use the degree of pleomorphism when grading many cancers: the more irregular the cells look, the more aggressive the tumor tends to be. The mechanism behind this irregularity involves the same kind of cytoskeletal disruption seen in bacteria, but in reverse. Instead of deliberately reorganizing their scaffolding, cancer cells lose control of it. Studies of oral cancer cells have linked chromosomal instability to defects in the cytoskeleton, where breakage-fusion-bridge cycles produce cells with abnormal chromosome counts and scrambled genomes.20PubMed. Chromosomal instability and cytoskeletal defects in oral cancer cells
Keratin, a structural protein found in epithelial cells, is another player. When keratin genes become fused or dysregulated in tumors, the resulting protein disrupts cytokinesis, the final stage of cell division. Cells fail to separate properly and end up with too many chromosomes, forming enlarged, oddly shaped cells. Research has shown that this keratin-driven instability also ramps up actin cytoskeleton remodeling, which increases the motility of the abnormal cells and could help tumors invade surrounding tissue.21bioRxiv. Cytoskeleton remodeling caused by keratin dysregulation triggers tumor aggressiveness via promoting genomic instability and cellular adaption In cancer, pleomorphism is not an adaptive survival strategy the way it is in microbes. It is a symptom of broken cellular machinery, but it still ends up conferring advantages like faster growth and easier spread.
Diagnostic Pitfalls
Pleomorphism creates real headaches in the diagnostic lab. When cells vary wildly in shape and size, it becomes harder to distinguish between benign and malignant tissue. This is particularly true in cytology, where clinicians examine individual cells rather than tissue architecture. In breast fine-needle aspiration biopsies processed with certain preparation methods, nuclear features like prominent nucleoli can appear more exaggerated than they would in traditional smears. Clumps of benign immune cells called epithelioid histiocytes can look atypical enough to raise false suspicion of cancer.22PubMed Central. Diagnostic dilemmas and pitfalls in ThinPrep® cytology of breast fine needle aspiration biopsy: Report of Six Cases with Histological Correlates
Similar problems arise in microbiology. If a bacterium has shifted to its L-form or coccoid state, it may no longer look like the species the lab technician expects to see. Standard identification methods that rely on cell shape, Gram-staining behavior, or growth on particular media can all fail when the organism is in a pleomorphic state. This is one reason infections with organisms like H. pylori or Mycobacterium tuberculosis can be missed on routine culture: the cells may be alive but hiding in a form that conventional tools do not recognize.
Nematode-Trapping Fungi and the Breadth of Morphological Change
Pleomorphism extends into some of the more exotic corners of biology. Nematode-trapping fungi are soil organisms that spend most of their lives as ordinary threadlike networks of hyphae. But when prey is nearby, they build elaborate trapping structures: adhesive knobs, constricting rings, or sticky nets, depending on the species. This morphological switch is not cosmetic. It represents a shift from a passive, nutrient-scavenging lifestyle to an active predatory one, and it requires remodeling of the cell wall, membrane, and internal signaling networks. In Arthrobotrys oligospora, deleting the gene for the PKA catalytic subunit made the fungus completely insensitive to the presence of nematodes, unable to sense its prey or build traps at all.23G3 Genes|Genomes|Genetics. The cAMP-PKA pathway regulates prey sensing and trap morphogenesis in the nematode-trapping fungus Arthrobotrys oligospora The finding reinforces how deeply shape is tied to function across biology. A cell’s form is not just packaging; it determines what the cell can do, how it interacts with its surroundings, and whether it lives or dies.