Escherichia coli, commonly known as E. coli, is a widespread bacterium found in various environments, including the gastrointestinal tracts of humans and warm-blooded animals. While many E. coli strains are harmless and even beneficial, some can cause illness. This single-celled organism serves as a significant subject in scientific research due to its simple structure and ease of cultivation, making it a valuable model for understanding fundamental biological processes.
The Basic E. coli Cell Shape
An individual E. coli cell is rod-shaped, also known as bacillus. These cells typically measure about 2.0 micrometers (µm) in length and between 0.25 to 1.0 µm in diameter, with a cell volume around 0.6 to 0.7 cubic micrometers. This cylindrical form is maintained by its multi-layered cell envelope.
Protruding from the cell surface are various appendages that facilitate interaction with its surroundings. Long, whip-like flagella enable movement through liquid environments via chemotaxis, allowing navigation towards beneficial substances or away from harmful ones. Shorter, hair-like pili or fimbriae are involved in attachment to surfaces, including host cells. Specialized sex pili facilitate genetic material transfer between bacterial cells.
Inside the E. coli Cell
E. coli’s internal organization is characteristic of a prokaryotic cell, lacking a membrane-bound nucleus and other complex organelles found in eukaryotic cells. Its genetic material, a single, circular DNA chromosome, is located in the nucleoid region of the cytoplasm. This DNA directs cell functions and protein creation.
Beyond the main chromosome, E. coli contains smaller, circular plasmids. These plasmids carry non-essential genes that provide survival advantages, such as antibiotic resistance, and can be transferred between bacteria. Ribosomes, responsible for synthesizing proteins, are found throughout the cytoplasm. The entire internal content is enclosed by a plasma membrane, acting as a selective barrier controlling what enters and exits the cell. Outside this membrane, a rigid cell wall composed of peptidoglycan provides structural integrity and maintains the cell’s shape.
How E. coli Forms Communities
E. coli can form complex, structured communities, primarily through biofilm formation. This process begins with cells adhering to a surface, facilitated by external structures like flagella, type I fimbriae, and curli. Initial attachment can be reversible, influenced by environmental changes like pH or temperature.
After initial adhesion, cells adhere to each other and secrete an extracellular matrix, composed of polysaccharides like cellulose and colanic acid. This matrix encases the bacterial community, protecting it from environmental stressors, including antibiotics and biocides. Colony formation is another collective arrangement, observed when E. coli grows on laboratory media like Luria-Bertani (LB) agar plates. These visible masses represent a higher level of organization, demonstrating the bacterium’s ability to thrive in aggregated forms.