Bacteria do not eat the way animals do. They have no mouths, no stomachs, and no digestive tracts. Instead, every bacterial cell feeds by absorbing dissolved molecules directly through its membrane, using an astonishing variety of molecular pumps, channels, and chemical tricks to pull in what it needs. The range of strategies bacteria have evolved to get nutrients and energy is broader than almost anything else in biology, spanning sunlight harvesting, rock oxidation, cooperative electron sharing, and even outright predation of other bacteria.
Getting Nutrients Through the Cell Wall
A bacterium’s outer surface is its entire interface with the world, and the way molecules cross that surface depends heavily on what kind of bacterium it is. Gram-negative bacteria have a double-membrane structure with an outer membrane that acts as a selective barrier. Small nutrients like sugars and amino acids pass through the outer membrane via barrel-shaped protein channels called porins. These porins are selective enough to control which molecules get in and how fast, creating a permeability barrier that also helps the cell resist toxic compounds.
Once a nutrient reaches the inner membrane, the cell faces a harder problem: moving molecules against a concentration gradient, from where they are scarce (outside) to where they are abundant (inside the cell’s cytoplasm). Bacteria solve this with dedicated transport proteins powered by different energy sources. Some use the energy stored in ATP. Others harness the natural flow of protons across the membrane. Still others use a clever chemical trick where a high-energy molecule from the cell’s own sugar-processing pathway donates a phosphate group to the incoming sugar as it crosses the membrane, chemically trapping it inside.
Glucose is the preferred carbon source for many bacteria, and the diversity of glucose transporters alone is remarkable. Cells can pull in this single sugar using ATP-powered transporters, proton-coupled carriers, sodium-coupled carriers, or the phosphotransferase system just described.
The Special Problem of Iron
Iron is essential for bacterial life because it sits at the heart of enzymes involved in energy production and DNA synthesis. But iron poses a unique challenge: under the oxygen-rich, near-neutral-pH conditions found in most living tissue and natural water, iron locks up into insoluble rust-like compounds that bacteria cannot absorb. On top of that, animal hosts deliberately sequester iron inside specialized proteins, keeping it out of bacterial reach as an immune defense.
Bacteria fight back with siderophores, small molecules they manufacture and release into their surroundings. Siderophores bind iron with extraordinary strength, prying it loose from host proteins or insoluble mineral deposits. The iron-loaded siderophore is then recognized by a dedicated receptor on the bacterial outer membrane and pulled inside through an energy-intensive process. In gram-negative bacteria, this import requires a protein complex called TonB-ExbB-ExbD, which sits in the inner membrane and uses the cell’s proton gradient to power the transport of siderophore-iron complexes across the outer membrane.
This system is so important that pathogenic bacteria often carry genes for multiple siderophore types, each optimized for different iron sources in the host. The siderophore system has even attracted attention from drug developers, who are designing antibiotics that hitch a ride on siderophores to sneak past bacterial defenses, exploiting the cell’s own iron hunger.
Digesting Food Outside the Cell
Many of the carbon sources bacteria encounter in nature are far too large to fit through any membrane channel. Polymers like cellulose, chitin (the material in insect exoskeletons and crustacean shells), and starch are long chains of sugar units that must be chopped into smaller pieces before a cell can absorb them. Bacteria handle this by secreting enzymes into their surroundings, breaking down the polymer externally and then importing the resulting fragments.
This extracellular digestion has interesting ecological consequences. When a bacterium releases an enzyme that chops up chitin in ocean sediment, the small sugar fragments it produces do not all end up back inside the cell that made them. Neighboring bacteria that never invested any energy in enzyme production can scavenge the released sugars, a phenomenon sometimes called cross-feeding. The bacterium doing the heavy enzymatic lifting, in a sense, sets a table that freeloaders can eat from too.
Energy From Sunlight
Plants are not the only organisms that harvest solar energy. Several groups of bacteria do it too, though often through fundamentally different chemistry. Cyanobacteria perform oxygenic photosynthesis much like plants, splitting water and releasing oxygen. But other photosynthetic bacteria use a different electron source entirely. Green sulfur bacteria, for instance, use hydrogen sulfide instead of water as their electron donor, producing sulfur rather than oxygen as a byproduct. This form of photosynthesis thrives in environments where oxygen is absent and sulfide is plentiful, like the deeper layers of stratified lakes and hydrothermal vents.
There is also a simpler, more widespread form of light harvesting that does not involve any of the elaborate photosynthetic machinery found in cyanobacteria or green sulfur bacteria. Many marine bacteria carry a protein called proteorhodopsin, a small light-driven proton pump embedded in the cell membrane. When sunlight hits proteorhodopsin, it pumps a proton out of the cell, building up the same kind of proton gradient that other bacteria generate by burning food. This gradient can then power nutrient uptake, maintain the cell’s electrical balance, and even fuel swimming. Proteorhodopsin-equipped bacteria are enormously common in the surface ocean, and research over the past two decades has shown that this light-harvesting strategy is especially useful during periods of carbon scarcity, giving cells a supplemental energy source when food is thin.
Energy From Rocks and Chemicals
Some bacteria have no need for organic food at all. Chemolithotrophs extract energy by oxidizing inorganic substances like ammonia, hydrogen sulfide, ferrous iron, or hydrogen gas. The electrons stripped from these inorganic donors are fed into an electron transport chain, generating a proton gradient and ultimately ATP, much like how animal cells generate energy from food-derived electrons, but with geological materials as the starting fuel.
These bacteria are not curiosities limited to extreme environments. Nitrifying bacteria that oxidize ammonia to nitrite, and then nitrite to nitrate, are critical players in the global nitrogen cycle and in wastewater treatment plants. Iron-oxidizing bacteria accelerate the corrosion of pipes and the weathering of mineral deposits. Sulfur-oxidizing bacteria in deep-sea hydrothermal vents form the base of entire ecosystems that run on chemical energy rather than sunlight.
Breathing Without Oxygen
Aerobic respiration, where oxygen serves as the final electron acceptor, yields the most energy per molecule of food. But oxygen is absent from vast swaths of Earth’s habitable environments: deep sediments, waterlogged soils, the interior of biofilms, and much of the deep ocean. Bacteria in these settings use alternative electron acceptors to keep their respiratory chains running.
Sulfate-reducing bacteria are among the most ecologically significant anaerobic respirers. They use sulfate (a compound abundant in seawater) as their terminal electron acceptor, producing hydrogen sulfide as a waste product. In oxygen-free marine sediments, sulfate reduction accounts for up to half of all organic matter breakdown in coastal and shelf ecosystems, making these bacteria central to both the carbon and sulfur cycles. Some sulfate reducers are metabolically flexible enough to also reduce nitrate to ammonia when sulfate runs low, giving them options in fluctuating environments.
When no external electron acceptor is available at all, bacteria can fall back on fermentation, a less efficient strategy that generates ATP through internal chemical rearrangements rather than by passing electrons to an outside molecule. Fermentation produces familiar end products like ethanol, lactic acid, and acetate. The energy yield is much lower than respiration, but it keeps the cell alive when nothing else will work. Researchers have continued to discover new fermentation pathways, including a relatively recently described route for making acetate from a common metabolic intermediate while generating ATP, showing that even well-studied organisms like gut bacteria still have metabolic surprises in store.
The boundary between respiration and fermentation is not always clean. Some bacteria, like certain species that can pass electrons to external minerals, generate most of their ATP through internal substrate-level reactions characteristic of fermentation, even though they depend on external electron acceptors to keep their overall metabolism balanced. The electron acceptor is necessary, but the ATP comes primarily from the older, simpler mechanism.
Biological Nanowires
One of the more striking discoveries in microbiology over the past two decades is that some bacteria can “breathe” solid minerals by sending electrons directly out of the cell and across considerable distances. The soil bacterium Geobacter sulfurreducens produces protein filaments, sometimes called nanowires, that are electrically conductive. These nanowires allow the cell to transfer electrons from its internal metabolism to iron oxide particles that it cannot bring inside. A mutant strain missing the genes for these filaments could still physically attach to iron oxide surfaces but could no longer reduce them, demonstrating that the nanowires are the functional conduit.
The molecular details of how electrons travel through these nanowires have been fleshed out in recent years. Inside the cell, small proteins in the space between the inner and outer membranes hand off electrons to larger proteins that form the nanowire filament. Different versions of these shuttle proteins contribute with varying efficiency, and even the least abundant one turns out to be the most effective electron donor, a finding that helps explain how the cell sustains electron transfer across a range of conditions. This extracellular electron transfer process is used by many bacteria, not just Geobacter, and the specific molecular machinery varies between species even when the overall strategy looks similar.
Feeding as a Team
Bacteria rarely live alone. In many environments, different species form metabolic partnerships where the waste products of one become the food of another. This cooperative metabolism, called syntrophy, allows reactions that would be energetically impossible for a single organism acting alone.
A well-studied example involves pairs of Geobacter species where one partner oxidizes an organic compound and passes the resulting electrons directly to the other partner, which uses them for its own metabolism. This direct interspecies electron transfer bypasses the need for a soluble chemical shuttle between cells, streamlining the energy exchange. In an especially creative version of this partnership, one species can hand electrons to another that uses them to fix carbon dioxide in the dark, essentially building organic molecules from COâ‚‚ without any light energy, powered entirely by electrons received from a partner cell.
Syntrophy also operates at larger scales with global consequences. In oxygen-free ocean sediments, consortia of methane-oxidizing archaea and sulfate-reducing bacteria work together to consume methane that would otherwise escape into the atmosphere. The archaea oxidize methane and transfer electrons to their bacterial partners, which use those electrons to reduce sulfate. Evidence points to direct interspecies electron transfer as the mechanism linking these two metabolisms, and the physical size of the consortia appears to be limited by how far electrons can travel between cells.
Finding Food in the First Place
Before a bacterium can absorb a nutrient, it often has to find it. Many bacteria are motile, propelling themselves with whip-like flagella, and they navigate chemical gradients through a behavior called chemotaxis. The classic model is Escherichia coli, which alternates between smooth “runs” in a roughly straight line and brief “tumbles” that randomly reorient the cell. When the cell detects that nutrient concentrations are increasing (because it is swimming toward a food source), it suppresses tumbling and extends its runs in the favorable direction. When concentrations drop, it tumbles more often, effectively randomizing its direction until it finds a better path.
What is surprising is that the bacterium is not actually comparing nutrient concentrations at its head versus its tail. The cell is far too small for that kind of spatial measurement to work. Instead, it compares the concentration it senses right now with what it sensed a fraction of a second ago, detecting a temporal gradient as it moves through space. A sudden increase in attractant concentration triggers a burst of smooth swimming, while a sudden decrease triggers tumbling. The cell then adapts to the new baseline concentration over time, resetting its sensitivity so it can detect the next change. This system is remarkably sensitive, allowing bacteria to respond to concentration changes of less than one percent.
Surviving Starvation
Nutrients are not always available, and bacteria have evolved ways to endure lean times. When starved of essential resources like nitrogen, many bacteria enter a dormant or near-dormant state. In E. coli, nitrogen starvation triggers the production of a signaling molecule called (p)ppGpp, which broadly reprograms the cell’s metabolism: growth slows, unnecessary protein production shuts down, and stress-resistance pathways activate.
One outcome of this starvation response is the formation of persister cells. These are not genetically different from the rest of the population; they are physiologically dormant cells that have temporarily stopped growing and become extremely tolerant to antibiotics and other stresses. Because most antibiotics work by disrupting active cellular processes like cell-wall construction or DNA replication, a cell that has paused those activities becomes effectively invisible to the drug. Persister cells are considered a major reason why some infections relapse after apparently successful antibiotic treatment: a small dormant fraction survives the drug, wakes up after treatment ends, and restarts the infection. Starvation is one of several environmental cues that push cells toward this persister state, with the (p)ppGpp signaling pathway playing a central coordinating role.
Bacteria That Feed Inside a Host
Some of the most successful bacterial feeding strategies involve intimate partnerships with larger organisms. Rhizobia, a group of soil bacteria, colonize the root nodules of leguminous plants like soybeans and clover. Inside these nodules, the bacteria convert atmospheric nitrogen gas into ammonia, a form of nitrogen the plant can use. In return, the plant supplies the bacteria with carbon compounds and all the other nutrients they need. The metabolic integration between plant and bacterium is remarkably tight, with dedicated transport systems shuttling sugars, amino acids, and metal ions across the membrane separating the two partners.
In the human gut, the situation is different but equally sophisticated. Members of the Bacteroidetes group, among the most abundant bacteria in the human intestine, specialize in digesting complex carbohydrates that our own enzymes cannot handle. They do this using coordinated gene clusters called polysaccharide utilization loci (PULs), which encode everything needed to sense, bind, and break down a specific type of complex sugar. A single Bacteroides species can carry dozens of different PULs, each targeting a different dietary fiber or host-derived glycan. These loci are tightly regulated so the cell only produces the relevant enzymes when the target sugar is actually present, conserving energy when that food source is absent. The deployment of different PULs shapes not just how individual species feed, but how entire gut microbial communities compete and coexist.
Predators and Plastic Eaters
Not all bacteria are content to absorb dissolved nutrients passively. Some are active predators that kill and consume other bacteria. Predatory bacteria like Bdellovibrio physically attack prey cells, drilling through their outer membrane and consuming the contents from inside. Other predatory species use contact-dependent mechanisms, requiring direct physical interaction with the prey cell to initiate killing. These predators acquire carbon, nitrogen, and other building blocks by dismantling their victims, and they are found across diverse environments from soil to the human gut.
At the other end of the feeding spectrum, some bacteria have evolved the ability to break down synthetic materials that have only existed for a few decades. Certain bacterial species produce enzymes that can degrade plastics like polyethylene terephthalate (PET) and polyethylene, converting these synthetic polymers into simpler molecules that can enter the cell’s normal metabolic pathways. The enzymes involved include esterases and oxidases that attack the chemical bonds holding the plastic polymer together. While this degradation is currently far too slow to solve the plastic pollution problem on its own, it demonstrates the remarkable metabolic adaptability of bacteria. Given enough evolutionary pressure and time, bacterial metabolism can expand to exploit almost any energy-containing material in the environment.
When One Strategy Is Not Enough
What makes bacterial feeding strategies so successful is not any single mechanism but the ability of many species to switch between strategies depending on conditions. A facultative anaerobe like E. coli will use oxygen when it is available, switch to nitrate reduction when oxygen disappears, and fall back on fermentation when no external electron acceptors are present at all. A marine bacterium carrying proteorhodopsin can supplement its carbon-based metabolism with light energy when the sun is shining and food is scarce. A sulfate reducer might pivot to nitrate when sulfate concentrations drop.
This metabolic flexibility extends to carbon sources too. Bacteria routinely monitor their surroundings for the most energetically favorable food and adjust enzyme production accordingly. When glucose is available, many species suppress the production of enzymes needed to metabolize other sugars, a prioritization system that ensures the cell always exploits the richest available food source first. Only when the preferred carbon source is exhausted does the cell invest in the machinery needed to process alternatives. The result is an organism that, despite being a single cell with no brain or nervous system, makes remarkably efficient foraging decisions driven entirely by molecular circuitry refined over billions of years of evolution.