E. coli is a single cell about two thousandths of a millimetre long, and it packs everything it needs for life into a layered rod. Most strains live harmlessly in our guts, but some cause urinary and food-borne infections, which is why its structure is studied so closely. Each part turns out to be a working machine, from a wall built by rotating proteins to a tail driven by a motor with gears.
The Outer Membrane Is a Tightly Packed Shield
E. coli is wrapped in an envelope with four layers: an outer membrane, a thin wall, a fluid-filled middle space, and an inner membrane.1PubMed Central. Envelope stress responses: balancing damage repair and toxicity The outer membrane is the first thing any chemical, drug, or immune attack meets. Its outer face is covered in lipopolysaccharide, or LPS, a fat-and-sugar molecule found on this group of bacteria.
Under very fine microscopes, the surface shows patches that each hold hundreds to thousands of LPS molecules.2Langmuir. High-Resolution Atomic Force Microscopy Studies of the Escherichia coli Outer Membrane: Structural Basis for Permeability Those patches pack so closely that they block many substances from getting in.3Langmuir. High-Resolution Atomic Force Microscopy Studies of the Escherichia coli Outer Membrane: Structural Basis for Permeability Between the patches, the membrane is mostly a network where proteins and LPS lock together in a rough honeycomb, with LPS-only patches in the gaps.4PubMed Central. Immobile lipopolysaccharides and outer membrane proteins differentially segregate in growing Escherichia coli
Textbook diagrams often show a sea of freely drifting lipids. In E. coli’s outer membrane, many parts are instead held nearly still. That rigidity makes the barrier effective, and it helps explain why some drugs that kill other bacteria cannot cross it.
The layer matters for illness too. LPS is what the human immune system reacts to strongly, and that reaction contributes to fever in serious infections. “Gram-negative” is the lab label for bacteria with this kind of outer membrane, and E. coli is the classic example.
How the Cell Wall Keeps E. coli Rod-Shaped
Beneath the outer membrane lies peptidoglycan, a mesh of sugar chains cross-linked by short protein pieces that acts as the cell wall. The cell’s contents push outward, and the mesh stops the cell from bursting. To stay a rod of even width, new wall material must be added evenly all around the cell as it grows.5PubMed Central. In Escherichia coli, MreB and FtsZ Direct the Synthesis of Lateral Cell Wall via Independent Pathways That Require PBP 2
A protein called MreB, a bacterial cousin of the actin in our muscles, is needed to keep the rod shape.6PubMed Central. The bacterial actin MreB rotates, and rotation depends on cell-wall assembly Blocking MreB with a drug makes cells bulge and lose their rod form.7PubMed Central. In Escherichia coli, MreB and FtsZ Direct the Synthesis of Lateral Cell Wall via Independent Pathways That Require PBP 2 MreB also rotates steadily around the long axis of the cell, and that rotation depends on wall building itself.8PubMed Central. The bacterial actin MreB rotates, and rotation depends on cell-wall assembly The spinning helps spread new wall evenly, which keeps the rod shape during growth.9PubMed Central. The bacterial actin MreB rotates, and rotation depends on cell-wall assembly Because MreB lays material along a twisting path, the cell slightly twists as it grows longer.10PubMed Central. Helical insertion of peptidoglycan produces chiral ordering of the bacterial cell wall
Shape and even wall building turn out to be separate jobs. A second protein, FtsZ, can also guide even wall building on its own, but only MreB decides whether the cell stays rod-shaped.11PubMed Central. In Escherichia coli, MreB and FtsZ Direct the Synthesis of Lateral Cell Wall via Independent Pathways That Require PBP 2 A wall-building enzyme called PBP 2 is part of the machinery both proteins use.12PubMed Central. In Escherichia coli, MreB and FtsZ Direct the Synthesis of Lateral Cell Wall via Independent Pathways That Require PBP 2 PBP stands for penicillin-binding protein, named because penicillin sticks to it.
That link explains how penicillin-type drugs work: they block the enzymes that build the wall mesh. Without a working wall, the growing cell can swell or burst.
The Space Between the Membranes
Between the inner and outer membranes is the periplasm, a gel-like space that also holds the wall. More than one fifth of the proteins E. coli makes are sent to its envelope.13PubMed. Folding mechanisms of periplasmic proteins So the cell needs a system to get those proteins folded and in place outside the main cell body.
A protein works only when it is folded into the right 3D shape, and misfolded proteins can clump and damage the envelope. The periplasm also lacks the cell’s usual energy supply, so the folding helpers there must work without it.
Two kinds of helpers do the folding. Chaperones guide folding or stop proteins clumping, without becoming part of them.14PubMed Central. Periplasmic Chaperones and Prolyl Isomerases Folding catalysts instead speed up specific folding steps, such as forming chemical bonds that lock a shape.15PubMed Central. Periplasmic Chaperones and Prolyl Isomerases Some outer-membrane proteins need both, with chaperones to carry them across and catalysts to form bonds.16PubMed. Folding mechanisms of periplasmic proteins
One helper, FkpA, is made in greater amounts during heat stress and can stop faulty proteins from clumping.17PubMed. Chaperone function of FkpA, a heat shock prolyl isomerase, in the periplasm of Escherichia coli Damage in this space also triggers alarms, which come up later in the article.
The Inner Membrane Powers the Cell
The inner membrane holds the cell’s breathing machinery, which passes electrons along to oxygen. This pumps protons out, building up proton pressure across the membrane.18PubMed Central. Aerobic respiratory chain of Escherichia coli is not allowed to work in fully uncoupled mode Protons flowing back in drive the making of ATP, the cell’s energy currency, and pull in nutrients.19PubMed Central. Aerobic respiratory chain of Escherichia coli is not allowed to work in fully uncoupled mode
In human cells, mitochondria do this proton pumping. E. coli has no mitochondria, so its inner membrane does the job.
E. coli has several of these breathing enzymes, and even a less efficient one still makes enough proton flow to power the cell.20PubMed Central. Aerobic respiratory chain of Escherichia coli is not allowed to work in fully uncoupled mode Having several helps because oxygen levels in the gut and outside change a lot. Different enzymes suit high or low oxygen, and switching between them lets the cell keep energy flowing as conditions change.
How E. coli Swims With a Rotary Motor
E. coli swims with several flagella, long whip-like tails turned by motors at their base. Each motor has a central spinning ring and about a dozen power units around it.21PubMed Central. Structure and Function of the Bi-Directional Bacterial Flagellar Motor Each power unit is anchored to the wall and lets protons through, turning that flow into a turning force.22PubMed Central. Structure and Function of the Bi-Directional Bacterial Flagellar Motor The power units themselves appear to spin, meshing with the ring like gears at about 6 to 1.23PubMed Central. Torque-generating units of the bacterial flagellar motor are rotary motors
The tail is a corkscrew, so spinning it pushes the cell along like a propeller. The motor can also reverse direction, which matters for steering.
The motor is strong for its size. Each power unit can push with a force of about 250 piconewton-nanometres before stalling.24PubMed Central. Direct Measurement of the Stall Torque of the Flagellar Motor in Escherichia coli with Magnetic Tweezers With a dozen units, the motor can spin the tail fast enough to move the cell many body lengths each second.
How E. coli Senses and Steers
E. coli moves in straight runs broken by tumbles that point it in a new direction. Sensor proteins send signals through a chain that switches the cell between running and tumbling.25PubMed Central. Escherichia coli chemotaxis is information limited When things are improving, it tumbles less and keeps going. When swarming across surfaces, it raises a protein that lowers tumbling, so it moves more steadily.26PubMed Central. Escherichia coli Remodels the Chemotaxis Pathway for Swarming
The switch connects directly to the motors. When they all turn one way, the tails bundle together and the cell runs. When some reverse, the bundle flies apart and the cell tumbles. The signal chain decides which way the motors turn.
The cell does not map its world. It compares conditions now with those a few seconds ago.
How Pili Help E. coli Stick and Cause Infections
Disease-causing E. coli use pili, hair-like surface fibres, to grip cells lining the urinary tract. Type 1 pili and P pili can both uncoil and stretch under force, then coil back.27PubMed Central. The biomechanical properties of E. coli pili for urinary tract attachment reflect the host environment Both unravel at a similar force, about 30 piconewtons.28PubMed Central. The biomechanical properties of E. coli pili for urinary tract attachment reflect the host environment But type 1 pili are stiffer and react far faster to a sudden pull.29PubMed Central. The biomechanical properties of E. coli pili for urinary tract attachment reflect the host environment They may therefore suit the bladder outlet, where urine flow is irregular, while P pili suit the steadier flow higher up toward the kidneys.30PubMed Central. The biomechanical properties of E. coli pili for urinary tract attachment reflect the host environment
Stretching can even strengthen the grip. Type 1 pili uncoil in a way tuned for grips that get stronger when pulled.31PubMed Central. Uncoiling Mechanics of Escherichia coli Type I Fimbriae Are Optimized for Catch Bonds So flushing urine can tighten rather than loosen their hold.
This matters because E. coli causes most urinary tract infections. Drinking fluids and urinating help flush bacteria out, but pili resist this. Research on blocking pili aims at new ways to prevent infection.
Where E. coli Keeps Its DNA and Builds Proteins
E. coli has no nucleus. Its single circular chromosome sits in an open region of the cell. Packing proteins fold it, creating long-range loops across most of the chromosome.32PubMed. Multiscale Structuring of the E. coli Chromosome by Nucleoid-Associated and Condensin Proteins One region near where copying ends is kept as a separate, tighter domain.33PubMed. Multiscale Structuring of the E. coli Chromosome by Nucleoid-Associated and Condensin Proteins A gene-silencing protein, H-NS, gathers the genes it controls into two compact clusters, and removing H-NS rearranges the whole chromosome.34PubMed Central. Chromosome organization by a nucleoid-associated protein in live bacteria
Our own DNA sits inside a nucleus behind a membrane. In E. coli, there is no such wall, so reading genes and making proteins can happen almost at once. That speed helps the cell react quickly.
Ribosomes, the protein-making machines, are mostly kept apart from the nucleoid.35PubMed Central. The spatial biology of transcription and translation in rapidly growing Escherichia coli In fast-growing cells, though, working ribosomes weave between and around DNA lobes.36PubMed Central. Organization of ribosomes and nucleoids in Escherichia coli cells during growth and in quiescence When growth stops or drugs block them, ribosomes gather at the cell ends.37PubMed Central. Organization of ribosomes and nucleoids in Escherichia coli cells during growth and in quiescence
How E. coli Divides and Shares Genes
E. coli divides by forming a ring of FtsZ protein that pinches the cell in two. Three Min proteins bounce back and forth between the cell ends, blocking the ring everywhere but the middle.38PubMed. Assembly dynamics of the bacterial MinCDE system and spatial regulation of the Z ring MinE forms its own ring near the middle that keeps the blocker away there.39PubMed. The MinE ring: an FtsZ-independent cell structure required for selection of the correct division site in E. coli The two daughter cells therefore come out equal in size.
In ideal lab conditions a cell can divide about every 20 minutes, so one cell could become millions overnight. That speed also explains why food left warm can grow risky bacteria.
E. coli can also pass extra DNA loops called plasmids to other cells in a process called conjugation, sometimes nicknamed bacterial sex.40PubMed Central. Structural bases for F plasmid conjugation and F pilus biogenesis in Escherichia coli Conjugation is a main way antibiotic resistance spreads.41PubMed Central. Structural bases for F plasmid conjugation and F pilus biogenesis in Escherichia coli The donor builds an F pilus, a hollow tube about 9 nanometres wide.42PubMed Central. Structure of the Bacterial Sex F Pilus Reveals an Assembly of a Stoichiometric Protein-Phospholipid Complex DNA has now been seen passing through this pilus between cells several micrometres apart.43PubMed Central. The mating pilus of E. coli pED208 acts as a conduit for ssDNA during horizontal gene transfer
How E. coli Survives Stress and Antibiotics
Heat, chemicals and other insults can damage the envelope and trigger stress responses that protect the cell.44PubMed Central. Envelope stress responses: balancing damage repair and toxicity One alarm, called sigma E, boosts folding helpers so outer-membrane proteins fold properly.45PubMed Central. Envelope stress responses: An interconnected safety net Another, Cpx, often works against it, cutting outer-membrane protein production.46PubMed Central. Envelope stress responses: An interconnected safety net A third system, Bae, switches on its own set of protective genes.47PubMed. A third envelope stress signal transduction pathway in Escherichia coli Cells lacking two alarms are more easily harmed than those lacking one.48PubMed. A third envelope stress signal transduction pathway in Escherichia coli
Some antibiotics damage DNA, which triggers a repair alarm that stops division.49PubMed Central. Proteomic Study of the Survival and Resuscitation Mechanisms of Filamentous Persisters in an Evolved Escherichia coli Population from Cyclic Ampicillin Treatment Cells keep growing without splitting, forming long filaments.50PubMed Central. Proteomic Study of the Survival and Resuscitation Mechanisms of Filamentous Persisters in an Evolved Escherichia coli Population from Cyclic Ampicillin Treatment Under ciprofloxacin, longer filaments go with longer stretched-out DNA.51PubMed Central. Escherichia coli survival in response to ciprofloxacin antibiotic stress correlates with increased nucleoid length and effective misfolded protein management In ordinary E. coli, though, most cells still die under ampicillin, and the few filaments usually burst.52PubMed Central. Proteomic Study of the Survival and Resuscitation Mechanisms of Filamentous Persisters in an Evolved Escherichia coli Population from Cyclic Ampicillin Treatment Some lab-evolved strains keep far more surviving filaments.
Surviving filaments matter because a few survivors can restart an infection after treatment.
Why E. coli Is a Workhorse for Science
Harmless lab strains of E. coli are used to make products such as insulin. Inserted genes can be broken by jumping DNA pieces in the cell’s genome.53PubMed Central. Reduced evolvability of Escherichia coli MDS42, an IS-less cellular chassis for molecular and synthetic biology applications A strain with all those pieces removed keeps engineered genes stable much longer.54PubMed Central. Reduced evolvability of Escherichia coli MDS42, an IS-less cellular chassis for molecular and synthetic biology applications E. coli also carries immune systems against viruses, including CRISPR, now a gene-editing tool.55PubMed Central. A functional selection reveals previously undetected anti-phage defence systems in the E. coli pangenome