Escherichia coli (E. coli) is a bacterium studied for many years, providing insight into the biology of single-celled organisms. A fundamental aspect of its biology is cell morphology, which is the study of the cell’s shape, size, and the arrangement of its parts. Understanding the structure of E. coli provides a baseline for recognizing how it functions and interacts with its environment.
The Archetypal Rod Shape
The most defining visual characteristic of E. coli is its bacillus, or rod, shape. These cells are cylindrical with hemispherical ends. On average, an individual bacterium measures about 2.0 micrometers (μm) in length and between 0.25 to 1.0 μm in diameter, giving it a cell volume of approximately 0.6 to 0.7 cubic micrometers.
While E. coli are single-celled organisms, they are often observed in pairs. This occurs briefly following cell division, before the two new daughter cells fully separate. The consistent rod shape under normal growth conditions is a hallmark of the species.
The Protective Cell Envelope
The cell envelope of E. coli is a complex, multi-layered structure that provides both protection and structural integrity. E. coli is classified as a Gram-negative bacterium, a distinction based on the composition of this envelope. This structure consists of three primary layers that mediate the cell’s relationship with its surroundings.
The innermost layer is the plasma membrane, which surrounds the cytoplasm. This membrane acts as a selective barrier, controlling the passage of nutrients, ions, and waste products. Just outside of this lies the peptidoglycan cell wall, situated in the periplasm. This layer, though thin in E. coli, is rigid and prevents the cell from bursting due to internal turgor pressure.
The outermost layer, a defining feature of Gram-negative bacteria, is the outer membrane. This membrane is composed of lipopolysaccharides (LPS) and serves as an additional protective barrier. It shields the cell from certain antibiotics, such as penicillin, and other harmful chemicals.
External Appendages for Movement and Attachment
Protruding from the surface of the E. coli cell are various appendages anchored to the cell envelope that facilitate interaction with its environment. The most prominent of these are flagella, which are long, whip-like filaments responsible for cell motility. These structures allow the bacterium to swim through liquid environments.
The movement driven by flagella is not random; it is directed by a process called chemotaxis. This system allows the cell to sense chemical gradients and rotate its flagella to move toward beneficial substances or away from toxins. This directed movement is a survival mechanism, enabling the cell to actively seek out favorable conditions.
In addition to flagella, E. coli cells are often covered in shorter, hair-like structures known as pili or fimbriae. These appendages are primarily involved in adhesion, enabling the bacterium to attach to surfaces, including the cells of a host organism. Certain specialized pili, known as conjugation pili, also play a role in transferring genetic material between bacterial cells.
Internal Organization
The interior of the E. coli cell, known as the cytoplasm, is a gel-like substance that fills the space within the plasma membrane. Unlike eukaryotic cells, such as those found in plants and animals, E. coli lacks a membrane-bound nucleus. Its genetic material is instead located in a specific region of the cytoplasm called the nucleoid.
The nucleoid contains a single, circular chromosome composed of approximately 4.6 million base pairs, which encodes all of the cell’s essential proteins. Scattered throughout the cytoplasm are thousands of ribosomes, the cellular machines responsible for protein synthesis. These structures translate genetic information from the chromosome into functional proteins.
The cytoplasm also contains various other molecules and plasmids. Plasmids are small, circular DNA molecules that can carry accessory genes, such as those for antibiotic resistance.
Variations in Form
Although E. coli is known for its consistent rod shape, its morphology can change in response to environmental cues and stresses, allowing it to survive challenging conditions. One such variation is filamentation, where the cell ceases to divide but continues to grow, resulting in an elongated, thread-like structure. This response is often triggered by DNA damage or exposure to certain antibiotics that interfere with cell division.
Another morphological change can occur if the cell wall is damaged. Without the rigid support of the peptidoglycan layer, the cell can lose its rod shape and become a spheroplast. This spherical form can be induced in a laboratory setting by treatment with enzymes like lysozyme, which degrades the peptidoglycan wall.
The morphology of E. coli can also differ depending on its lifestyle. Cells growing within a biofilm—a community of bacteria attached to a surface—can exhibit different shapes and sizes compared to their free-swimming counterparts.