Several lineages of prokaryotes are genuinely multicellular, with differentiated cell types, coordinated behavior, and physical connections between cells. The textbook distinction that prokaryotes are unicellular while eukaryotes can be multicellular is a simplification that breaks down under scrutiny. From cyanobacteria that split labor between photosynthesis and nitrogen fixation, to magnetotactic bacteria that never exist as single cells, the prokaryotic world hosts more multicellular diversity than most biology courses suggest.
Cyanobacteria With Specialized Cells
The best-studied example of prokaryotic multicellularity is found in filamentous cyanobacteria, particularly species of the genus Anabaena. These organisms grow as long chains of cells, and when starved of nitrogen, roughly 5 to 10 percent of the cells along each filament transform into a completely different cell type called a heterocyst.1PubMed. Heterocyst differentiation and pattern formation in cyanobacteria: a chorus of signals Heterocysts lose the ability to divide but gain the ability to pull nitrogen gas from the atmosphere, something the ordinary vegetative cells cannot do because their oxygen-producing photosynthesis would destroy the enzyme responsible. The two cell types depend on each other: vegetative cells provide sugars from photosynthesis, while heterocysts supply fixed nitrogen back to the filament. This is genuine division of labor between permanently distinct cell types, one of the hallmarks biologists look for when defining multicellularity.
The heterocysts do not appear randomly. They space themselves in a quasi-regular pattern along the filament, ensuring every vegetative cell is within a few neighbors of a nitrogen source.2PubMed Central. Formation and maintenance of nitrogen-fixing cell patterns in filamentous cyanobacteria This spatial patterning is maintained by chemical signaling between cells, a process that looks remarkably like the patterning mechanisms that shape tissues in animal and plant embryos.
How Cyanobacterial Cells Talk to Each Other
Multicellularity requires more than just cells sticking together. It requires communication, and filamentous cyanobacteria have evolved a dedicated infrastructure for it. Adjacent cells are linked by structures called septal junctions, proteinaceous tubes that thread through tiny pores in the shared cell wall. Each septum between two cells contains dozens of these nanopores, roughly 20 nanometers wide, and the junctions allow molecules to pass directly from one cell’s interior to the next by simple diffusion.3PubMed. Cell-cell communication through septal junctions in filamentous cyanobacteria4PubMed Central. Molecular Diffusion through Cyanobacterial Septal Junctions
Detailed structural work has revealed that each septal junction is more sophisticated than a simple hole. The tube itself is about 11 nanometers wide with a 7-nanometer lumen, made of protein rather than membrane. On each cytoplasmic side sits a cap structure with five-fold symmetry that can close the junction reversibly under stress.5Cell. Architecture and Mechanism of Cyanobacterial Septal Junctions This gating ability means the filament is not just passively connected. Cells can selectively shut down communication when damaged, protecting the rest of the organism. The parallels to gap junctions in animal tissues and plasmodesmata in plant tissues are hard to miss, though the cyanobacterial version evolved independently.
Myxobacteria and Cooperative Predation
Myxobacteria take a different route to multicellularity. Rather than forming permanent filaments, species like Myxococcus xanthus live as swarms of individual rod-shaped cells that move, hunt, and develop as a coordinated group. Their feeding strategy is genuinely social: swarms of cells collectively secrete digestive enzymes that lyse prey bacteria, then share the released nutrients.6PubMed Central. Myxobacteria: Moving, Killing, Feeding, and Surviving Together This cooperative hunting only works at high cell densities, since a lone cell cannot produce enough enzymes to break down prey effectively.
The most striking multicellular behavior in myxobacteria appears when food runs out. Under starvation, roughly 100,000 cells aggregate into mound-shaped structures called fruiting bodies, inside which some cells differentiate into tough, dormant spores.7PubMed Central. Aggregation during fruiting body formation in Myxococcus xanthus is driven by reducing cell movement8PubMed Central. β-d-Allose Inhibits Fruiting Body Formation and Sporulation in Myxococcus xanthus Not all cells become spores. Many die during the process, effectively sacrificing themselves so the survivors can persist. When prey bacteria are present, the timing of fruiting body formation responds to the availability of prey, with development triggered specifically when prey levels drop.9PubMed Central. Multicellular Development in Myxococcus xanthus Is Stimulated by Predator-Prey Interactions This conditional, context-dependent development is far more complex than a simple on-off switch.
Streptomyces and Fungus-Like Growth
If you looked at a Streptomyces colony under a microscope without being told what it was, you might guess you were seeing a fungus. These soil bacteria grow as a branching network of long, multinucleated filaments called a mycelium, with occasional cross-walls separating extended compartments.10PubMed Central. Subcompartmentalization by cross-membranes during early growth of Streptomyces hyphae When conditions are right, parts of the colony differentiate into aerial hyphae that rise above the surface and eventually septate into chains of single-nucleoid spores.11Trends in Microbiology. Regulation of Streptomyces development: reach for the sky!
This life cycle involves at least two functionally distinct tissue-like phases: a feeding substrate mycelium that absorbs nutrients, and reproductive aerial hyphae that produce spores. The transition between these phases is tightly regulated. Specific gene families encode small secreted proteins that coat the aerial hyphae, making them hydrophobic enough to break through the water-air interface. Deletion of key genes in this family severely delays aerial growth, though the spores that eventually form are normal.12PubMed Central. A novel class of secreted hydrophobic proteins is involved in aerial hyphae formation in Streptomyces coelicolor by forming amyloid-like fibrils The parallels to fungal development are a textbook case of convergent evolution: two completely unrelated lineages arrived at strikingly similar solutions for growing as multicellular filaments in soil.
Bacteria That Are Never Single Cells
Perhaps the most remarkable prokaryotic multicellular organisms are the multicellular magnetotactic bacteria, or MMBs. These are currently the only known bacteria that lack a unicellular stage entirely.13PLOS Biology. Multicellular magnetotactic bacteria are genetically heterogeneous consortia with metabolically differentiated cells Each organism is a hollow sphere or ellipsoid made up of roughly 50 tightly coordinated cells, typically 3 to 6 micrometers across, found in marine sediments.14PubMed Central. Giant multicellular magnetotactic prokaryotes in marine sediments
The cells in an MMB are arranged in a single polarized layer around the hollow interior. When the organism reproduces, all of the cells divide synchronously, the sphere elongates into an ellipsoid, and then it splits into two equal daughter spheres with a twisting motion at the equatorial plane.15PubMed. Multicellular life cycle of magnetotactic prokaryotes This is whole-organism binary fission, not individual cell reproduction followed by reaggregation. The organism is multicellular from birth to death.
Coordination extends to movement. Thousands of flagella on the outer surface of the constituent cells beat together to propel the sphere through sediment along geomagnetic field lines, and the entire organism responds as a unit to mechanical, chemical, magnetic, and light stimuli.16PubMed. Juxtaposed membranes underpin cellular adhesion and display unilateral cell division of multicellular magnetotactic prokaryotes Recent genetic analysis has added another surprise: the cells within an MMB are not all genetically identical. They form a genetically heterogeneous consortium with metabolically differentiated cells, blurring the line between a single multicellular organism and a tightly integrated microbial community.13PLOS Biology. Multicellular magnetotactic bacteria are genetically heterogeneous consortia with metabolically differentiated cells
Cable Bacteria and Centimeter-Scale Electricity
Cable bacteria are filamentous organisms found in aquatic sediments that have evolved what might be the most exotic form of multicellularity known. Individual filaments span centimeters in length, threading from oxygen-rich surface layers down into oxygen-free deeper sediment. Cells at the bottom oxidize sulfide (extracting electrons), while cells at the top reduce oxygen (consuming electrons), and the electrons travel the full length of the filament through highly conductive protein fibers embedded in the cell envelope.17PubMed Central. Long-distance electron transport in individual, living cable bacteria
The distances involved are extraordinary by biological standards. Electrical currents have been measured running across filaments more than 10 millimeters long, passing through a sequence of over 2,000 cells.18Nature Communications. A highly conductive fibre network enables centimetre-scale electron transport in multicellular cable bacteria This vastly exceeds the micrometer-scale electron transport previously documented in other bacteria. Laser-cutting a cable bacterium filament instantly breaks down the electron-transport gradient, confirming that the current flows continuously through the living cells rather than through the surrounding environment.17PubMed Central. Long-distance electron transport in individual, living cable bacteria The cells in a cable bacterium depend on each other in the most literal way imaginable: cells at opposite ends of the filament access different chemical resources and share the electrons between them, making neither end self-sufficient.19PubMed. Cable Bacteria Take a New Breath Using Long-Distance Electricity
A Recently Discovered Underground Bacterium
New forms of prokaryotic multicellularity continue to turn up. In 2022, researchers reported a bacterium called HS-3, isolated from an underground stream, that displays a particularly organized multicellular structure. HS-3 self-organizes its filamentous cells into a layered colony with the physical properties of a liquid crystal. As the colony matures, it forms a semi-closed sphere that encloses clusters of smaller daughter cells and selectively releases them when the sphere contacts water.20eLife. Novel multicellular prokaryote discovered next to an underground stream Genetically, HS-3 belongs to the family Neisseriaceae, a group better known for the pathogens that cause meningitis and gonorrhea. Its multicellular lifestyle was a complete surprise and suggests that bacterial multicellularity may be more widespread than current surveys indicate, since the environments where these organisms thrive (underground aquifers, deep sediments) are undersampled.
Where Biofilms Fit In
If multicellularity means “multiple cells cooperating in a differentiated structure,” then what about biofilms? Almost all bacteria can form biofilms, those slimy mats on surfaces where cells embed themselves in a self-produced matrix. Biofilms do have some properties of multicellular organisms. Cells within them take on different metabolic roles depending on their position, and the community responds to environmental stimuli through coordinated regulatory mechanisms.21PubMed Central. Three faces of biofilms: a microbial lifestyle, a nascent multicellular organism, and an incubator for diversity
But most biologists draw a distinction between biofilms and the examples described above. Biofilm cells are generally capable of independent life; they can detach and survive alone. The cells are not typically connected by specialized physical structures for direct molecular exchange. And while there is spatial differentiation, it usually arises from gradients in oxygen and nutrients rather than from a genetic developmental program. The organisms described in previous sections go further: heterocysts cannot revert to vegetative cells, MMB cells never exist alone, and cable bacteria cells at opposite ends of a filament literally cannot survive without each other. That said, the boundary is blurry, and some researchers view biofilms as “nascent multicellular organisms” that represent the early stages of a continuum.
Programmed Cell Death as a Multicellular Trait
One of the more unexpected lines of evidence for prokaryotic multicellularity comes from programmed cell death. In eukaryotes, the controlled suicide of certain cells (apoptosis) is a defining feature of complex multicellular development. It turns out that bacteria have their own versions. In Escherichia coli, a toxin-antitoxin system called mazEF can trigger cell death under stress, removing damaged individuals from the population.22PubMed Central. Bacterial programmed cell death and multicellular behavior in bacteria In Bacillus subtilis, two separate gene sets mediate the killing of a subpopulation of cells during sporulation, freeing resources for the survivors.22PubMed Central. Bacterial programmed cell death and multicellular behavior in bacteria
Programmed cell death only makes evolutionary sense when the dying cell’s sacrifice benefits genetically related neighbors. In a purely solitary organism, it would be a dead end. The existence of these systems in bacteria is taken by some researchers as evidence that bacterial populations function, at least sometimes, as multicellular units where individual fitness is subordinate to group fitness.23PubMed. Death’s toolbox: examining the molecular components of bacterial programmed cell death
Why Prokaryotic Multicellularity Did Not Produce Animals or Plants
Given all these examples, a natural question is why prokaryotic multicellularity stayed relatively simple. Cyanobacterial filaments, myxobacterial fruiting bodies, and cable bacteria are impressive, but nothing in the prokaryotic world approaches the complexity of a fruit fly, a fern, or even a sponge. The most widely cited explanation centers on energy. Prokaryotic cells generate energy across their outer membrane, which means the ratio of energy-producing membrane to genome stays roughly fixed as the cell grows. Without a way to dramatically increase energy output per gene, prokaryotic genomes are effectively capped in size.24Nature. The energetics of genome complexity
The endosymbiotic event that gave rise to mitochondria changed that equation for eukaryotes, allowing a roughly 200,000-fold expansion in the number of genes a cell could express.24Nature. The energetics of genome complexity With vastly larger genomes came the regulatory complexity needed to build hundreds of different cell types, fold tissues into organs, and coordinate development across trillions of cells. Prokaryotes, without that bioenergetic leap, can manage a handful of cell types and modest body plans, but they hit a ceiling. The multicellularity they achieve is real, but it is constrained by the fundamental architecture of their cells.
Deep Evolutionary Roots
Prokaryotic multicellularity is not a recent invention. Phylogenetic analyses of cyanobacteria indicate that multicellularity arose in the group before the Great Oxidation Event roughly 2.4 billion years ago, making it one of the oldest evolutionary innovations on the planet.25PubMed Central. Cyanobacteria and the Great Oxidation Event: evidence from genes and fossils The transition to multicellularity may have been a key driver of cyanobacterial diversification, creating a new unit for natural selection to act upon and enabling the variety of forms seen in modern cyanobacteria.
The selective advantages of multicellularity for prokaryotes are broad: better resistance to physical and chemical stress, improved access to resources, protection from predation, and the opportunity for cell specialization. Predation pressure appears to be a particularly strong driver. Studies have shown that filamentous bacteria resist engulfment by predatory protists more effectively than single cells, and that co-culturing bacteria with predators selects for longer filaments over time.26PubMed Central. On The Evolution of Bacterial Multicellularity In other words, being bigger and connected provided a survival advantage long before eukaryotes arrived on the scene.
Nanotubes Between Unrelated Bacteria
Beyond the specialized connections seen in cyanobacterial filaments, some bacteria form physical bridges to their neighbors even across species lines. Researchers working with Bacillus subtilis discovered that cells extend membrane-derived nanotubes to adjacent cells, creating a route for cytoplasmic molecules to move between them. These nanotubes also formed between B. subtilis and Staphylococcus aureus, and even between B. subtilis and the distantly related Escherichia coli.27Cell. Intercellular Nanotubes Mediate Bacterial Communication In separate experiments, bacteria that were auxotrophic (unable to make certain amino acids) were able to survive when connected by nanotubes to cells that could supply the missing nutrients, with tubes averaging about 80 nanometers in diameter and spanning up to 14 micrometers between cells.28Nature Communications. Metabolic cross-feeding via intercellular nanotubes among bacteria
This interspecies molecular sharing is not multicellularity in the developmental sense, but it demonstrates that the physical infrastructure for cell-cell cooperation is widespread in the bacterial world. It also raises the possibility that some natural microbial communities are more physically integrated than we assumed, blurring the line between a collection of cooperating species and a loosely organized multicellular unit.
Archaea Getting in on the Act
The discussion of prokaryotic multicellularity has traditionally focused on bacteria, but archaea may have their own version. Recent work with haloarchaea found that when cells were subjected to mechanical compression, they stopped dividing but continued to grow. After about 12 hours, something unexpected happened: multiple simultaneous division events carved each large cell into compartments, producing a connected monolayer of cells resembling the tessellation patterns found in plant leaf tissue. The resulting structures preserved the archaeal cell-surface layer between cells and exhibited material properties typical of eukaryotic epithelial tissues.29bioRxiv. Tissue-Like Multicellular Development Triggered by Mechanical Compression in Archaea This work is still in preprint and needs independent confirmation, but if it holds up, it means both major prokaryotic domains, bacteria and archaea, have the capacity for tissue-like multicellular organization, and the trait may be far more deeply rooted in the tree of life than anyone expected.
Engineering Synthetic Division of Labor
The existence of natural prokaryotic multicellularity has inspired synthetic biologists to deliberately engineer division of labor into bacterial communities. The idea is straightforward: instead of forcing a single microbial strain to carry out an entire complex metabolic pathway, you split the pathway across multiple specialized subpopulations. Each subpopulation handles one portion of the work, reducing the metabolic burden on any individual cell and potentially increasing overall efficiency.30PubMed Central. Investigating The Potential of Division of Labour in Synthetic Bacterial Communities for the Production of Violacein This approach draws directly on the same principle that makes natural multicellularity advantageous: cells that specialize can outperform cells that try to do everything themselves. The difference is that synthetic biologists can design the division of labor from scratch, choosing which steps to assign to which population, rather than waiting for evolution to stumble onto a solution.