Escherichia coli (E. coli) is a Gram-negative, facultatively anaerobic, rod-shaped bacterium that normally lives in the intestines of humans and animals. It was first discovered in 1885 by Theodor Escherich, a German-Austrian pediatrician, who used his own anaerobic culture methods along with Hans Christian Gram’s newly developed staining technique to isolate bacteria from infant fecal samples. He identified the common colon bacillus and named it Bacterium coli commune. Later, Castellani and Chalmers proposed the name Escherichia coli in 1919 to honor Escherich, and the name became official in 1958. [Reference]

Most E. coli strains are harmless and even helpful (e.g., aiding digestion and making vitamin K), though some cause food poisoning, UTIs, or diarrhea [Reference]. E. coli grows rapidly often doubling in about 20 minutes under ideal conditions so it’s a key model organism in biology and biotechnology.

1. Capsule 

The capsule is the outermost covering present in some bacteria. It appears as a slimy or sticky layer outside the cell wall and is sometimes referred to as the glycocalyx when it is loosely attached. 

The capsule is mainly composed of polysaccharides (long chains of sugar molecules), such as glucose, galactose, rhamnose, mannose, or sialic acid. In some bacteria, however, the capsule may also contain proteins or be made of a combination of sugars and proteins (glycoproteins). An unusual example is Bacillus anthracis (the anthrax bacterium), whose capsule is made of a polypeptide instead of sugars. Because of its composition, the capsule is usually thick, sticky, and jelly-like in texture.

The capsule plays several important roles in bacterial survival. It prevents the cell from drying out (desiccation), acts as a protective shield against the host’s immune defenses (for example, by preventing white blood cells from engulfing the bacterium), and helps the bacterium stick to surfaces or host tissues. In many disease-causing bacteria, the capsule is considered a virulence factor because it increases their ability to cause infections and survive inside the host.

2. Cell Wall 

The cell wall is a strong and rigid layer that surrounds the cell membrane in most bacteria, giving the cell its shape and protection. It is mainly made of peptidoglycan, a unique molecule composed of sugars (N-acetylglucosamine and N-acetylmuramic acid) cross-linked with short chains of amino acids. This structure makes the wall tough and prevents the cell from bursting when water enters by osmosis. 

The thickness and arrangement of the cell wall differ between two main groups of bacteria: Gram-positive bacteria have a thick peptidoglycan layer, which retains the purple stain in Gram staining, while Gram-negative bacteria have a thinner peptidoglycan layer plus an outer membrane containing lipopolysaccharides (LPS) that provide extra protection and can trigger strong immune responses. 

Because of these differences, the cell wall is not only important for survival and shape but also plays a role in how bacteria interact with their environment, resist antibiotics, and cause disease. 

3. Cell Membrane 

The cell membrane, also called as plasma membrane, is a thin and flexible layer located just beneath the cell wall in bacteria. It is made up of a phospholipid bilayer with embedded proteins, and unlike eukaryotic cells, most prokaryotic membranes do not contain sterols such as cholesterol, though some exceptions, like Mycoplasma, have sterol-like molecules. The membrane acts as a protective barrier, controlling the entry of nutrients and the removal of wastes, while maintaining the internal environment of the cell through its property of selective permeability. 

It also plays a central role in metabolism, since bacteria lack membrane-bound organelles; for example, in aerobic bacteria, the plasma membrane is the site of cellular respiration, while in photosynthetic bacteria, it contains pigments for photosynthesis [References]. In addition, the proteins in the membrane help transport molecules and allow the cell to sense and respond to signals from its surroundings. Thus, the bacterial cell membrane is not just a boundary but an active center for energy production, transport, and communication. 

4. Flagella 

Flagella are long, whip-like structures that extend from the surface of many bacterial cells and are used mainly for movement. They act like tiny propellers, rotating to push the bacterium forward or backward in a liquid environment. Each flagellum is made of a protein called flagellin and has three main parts: the filament (the long tail-like part), the hook (a curved structure that connects the filament to the base), and the basal body (a motor-like structure anchored in the cell wall and membrane). The basal body uses energy from the cell, often in the form of a proton gradient (proton motive force), to rotate the flagellum. 

Depending on the type of bacterium, flagella may be present in different numbers and arrangements, for example, a single flagellum at one end (monotrichous), a cluster at one end (lophotrichous), one at both ends (amphitrichous), or many spread all over the surface (peritrichous). Flagella not only help bacteria move but also guide them toward food or away from harmful substances, a process known as chemotaxis. 

5. Fimbriae 

Fimbriae are short, thin, hair-like projections found on the surface of many bacterial cells. Unlike flagella, they are not used for movement but mainly for attachment. 

They are made of protein called as pilin and occur in large numbers, often covering the entire bacterial surface. Fimbriae help bacteria stick to surfaces, tissues, or each other, which is important for forming biofilms and for colonization during infection. 

For example: Disease-causing bacteria use fimbriae to attach firmly to host cells, preventing them from being washed away by body fluids. Because of this role, fimbriae are an important factor in bacterial pathogenicity (ability to cause disease). 

6. Pili 

Pili (singular: pilus) are hair-like structures found on the surface of many bacterial cells, similar in appearance to fimbriae but usually longer and fewer in number. They are made of a protein called pilin. Unlike fimbriae, pili are not primarily for attachment to surfaces; instead, their main role is in conjugation, a process where one bacterium connects to another to transfer genetic material, usually in the form of plasmids. This is why they are often called sex pili. 

Through this exchange, bacteria can share genes that may provide advantages, such as antibiotic resistance. Some pili also help bacteria attach to specific host tissues or surfaces, contributing to infection. 

In summary, pili play a key role in bacterial communication, genetic exchange, and sometimes adhesion, making them an important feature of many prokaryotic cells.

1. Cytoplasm 

The cytoplasm is a jelly-like fluid that fills the inside of the cell. It is mostly water but also contains dissolved salts, enzymes, and many molecules needed for cell functions. All the cell’s internal components, like ribosomes and the nucleoid, are suspended in the cytoplasm. It is the main site where metabolic reactions, such as breaking down nutrients and producing energy, occur. 

2. Nucleoid

Instead of a nucleus, bacteria have a nucleoid, which is simply a region in the cytoplasm where the DNA is located. The bacterial DNA is usually a single, circular chromosome that contains all the genetic information needed for growth, reproduction, and survival. Since it isn’t enclosed by a membrane, the DNA directly interacts with the cytoplasm and ribosomes. 

3. Plasmid 

In addition to the main chromosome, many bacteria also contain small, circular DNA molecules called plasmids. Plasmids carry extra genes that are not essential for basic survival but provide special advantages, such as antibiotic resistance. Plasmids can even be shared between bacteria, helping them adapt quickly to new environments. 

4. Ribosomes

Bacteria contain ribosomes, which are smaller than those in eukaryotic cells (70S type). Ribosomes are responsible for protein synthesis—they read the genetic code from the DNA (via RNA) and assemble proteins, which are crucial for all cellular activities. 

5. Storage Granules & Inclusion Bodies 

Some bacteria store nutrients or energy in special forms like glycogen, lipids, or polyphosphate granules. These storage bodies help the cell survive when food is scarce. 

6. Mesosome

Mesosomes are folded parts of the cell membrane that appear as inward projections into the cytoplasm. They were once thought to play important roles in respiration, DNA replication, and cell division, but modern research shows that many mesosomes may actually be artifacts caused during sample preparation for electron microscopy. [Reference]