Escherichia coli (E. coli) is a rod-shaped bacterium found in the lower intestine of warm-blooded animals. Many strains are harmless and part of the normal gut flora, helping produce vitamin K2 or preventing pathogenic bacteria from colonizing the intestine. The bacterium is also used in microbiology and biotechnology research because it is easy to grow, has been studied extensively, and reproduces rapidly. While many strains are beneficial, certain types can cause illness, making an understanding of its cellular structure relevant.
The E. coli Cell Envelope
The protective outer layers of an E. coli cell are the cell envelope, a multi-layered structure characteristic of a Gram-negative bacterium. Some strains possess an outermost polysaccharide capsule, a gelatinous layer that helps the bacterium adhere to surfaces and provides protection from the host’s immune system. This capsule is a factor in the ability of certain strains to cause disease.
Beneath the capsule, or as the outermost layer in strains that lack one, is the outer membrane. This membrane is composed of phospholipids, proteins, and its most notable component, lipopolysaccharide (LPS). LPS is an endotoxin that can trigger a strong immune response in humans. The outer membrane acts as a selective barrier, containing proteins called porins that form channels to allow specific molecules to pass into the cell.
Between the outer and inner membranes lies the periplasmic space, a gel-like compartment containing a thin layer of peptidoglycan. This peptidoglycan wall gives the bacterium its structural integrity and shape, protecting it from bursting due to internal water pressure. The periplasm is filled with enzymes involved in nutrient breakdown, transport, and cellular maintenance.
The innermost layer of the envelope is the plasma membrane, or inner membrane. This structure is a phospholipid bilayer with numerous embedded proteins, similar to cell membranes in other organisms. The plasma membrane is responsible for controlling the passage of nutrients and waste. It also houses the machinery for cellular respiration and energy production, a role mitochondria perform in eukaryotic cells.
Cytoplasmic Contents and Genetic Material
The interior of the E. coli cell is filled with the cytoplasm, a dense substance composed of water, salts, and organic molecules. This internal environment suspends the cell’s components and is the site of many metabolic reactions. Unlike eukaryotic cells, E. coli houses its genetic material in a region of the cytoplasm known as the nucleoid.
The nucleoid contains the bacterium’s genome, which is a single, circular chromosome of double-stranded DNA. This large DNA molecule is highly compacted and organized by associated proteins, allowing it to fit within the cell. The genetic information in this chromosome directs all of the cell’s life processes.
Scattered throughout the cytoplasm are thousands of ribosomes, the molecular machines for protein synthesis. In E. coli, these are the 70S type, which are smaller than those in eukaryotes. Ribosomes read the genetic instructions carried by messenger RNA (mRNA) and translate them into proteins that perform many functions within the cell.
Many E. coli cells also contain plasmids, which are small, circular DNA molecules that replicate independently of the main chromosome. Plasmids are not required for basic survival but often carry genes that provide a selective advantage, such as antibiotic resistance or toxin production. These genetic elements can be transferred between bacteria, contributing to the spread of such traits.
External Structures for Motility and Adhesion
Extending from the surface of the E. coli cell are appendages for movement and interaction with its environment. The most prominent of these are flagella, long, whip-like structures that enable the bacterium to swim. An E. coli flagellum is a motor-like assembly composed of a basal body in the cell envelope, a hook, and a long filament made of the protein flagellin. The rotation of these flagella propels the cell through liquids.
Many E. coli strains are covered with shorter, hair-like appendages called pili or fimbriae. These structures are involved in adhesion, allowing the bacteria to attach to surfaces, including the cells of a host organism. This ability to adhere is a factor for colonization and a first step in causing an infection.
A specific type of pilus, the sex pilus or F-pilus, has a specialized role in a process called conjugation. The sex pilus is longer than common pili and forms a direct physical connection with another bacterial cell. Through this connection, genetic material like plasmids can be transferred from a donor to a recipient. This horizontal gene transfer is a primary way that bacteria share genetic information.