Bacterial Cell Structure: Complete Guide to Prokaryotic Cell Organization and Components

Understanding bacterial cell structure is fundamental to microbiology and cell biology. Bacteria are microscopic, unicellular prokaryotic organisms with a simple yet highly organized cellular architecture. These prokaryotic cells differ significantly from eukaryotic cells, lacking a membrane-bound nucleus and complex organelles. The cell wall, cell membrane, cytoplasm, ribosomes, nucleoid region, and other external appendages are all important parts of a bacterial cell.

This comprehensive guide explores the complete organization of bacterial cells, including their morphology, structural features, and functional characteristics.

To help people understand how prokaryotic organisms are organized, the structure of a bacterial cell is broken down into these three main groups. Each component plays a vital role in bacterial cell function, survival, and pathogenicity. The study of bacterial cell morphology and anatomy reveals the intricate design of these simple yet highly efficient microorganisms.

Structurally, a bacterial cell consists of three categories of structures, namely,

1. Structures of the external side of the cell wall.
2. Cell wall
3. Structures of the Internal side of the cell wall

External Bacterial Cell Structures and Appendages

The bacterial cell Structures at the external side of the cell wall include flagella, fimbriae (pili), and capsule (slime layer).

1. Flagella: Bacterial Cell Motility Structures

Flagella are thin, hair-like appendages that originate from a granular structure, the basal body, which is present just beneath the plasma membrane. They are composed of protein, “flagellin.” The flagellin molecules are forming a single cylindrical filament.

Flagella are present in eubacteria. The bacilli possess flagella. The cocci do not possess flagella. They are responsible for the motility of bacteria.

Based on the arrangement of flagella, four types of arrangements are recognized. They are “Lophotrichous, Amphitrichous, Monotrichous, and Peritrichous” (the shortcut for easy recollection is LAAMP).

1. Lophotrichous: A tuft of flagella attached to one side of the cell.
2. Atrichous: Flagella are totally absent
3. Amphitrichous: Two tufts of flagella (or) a single flagellum on either end of the cell.
4. Monotrichous: A single flagellum is found on one side of the cell.
5. Peritrichous: Many flagella are found all over the cell surface.

Structurally, a flagellum is composed of three parts:

• Basal body 
• Hook         
• Filament

a) Basal body

It is the most complex part of the flagellum. The basal body is located entirely within the cell envelope and measures about 27 nm in length. The basal body consists of a small central rod inserted into a system of rings. The system of rings, however, differs between gram-positive and gram-negative types of bacteria.

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b) Hook

It is the part that lies between the filament and the basal body of the flagellum. The hook is slightly wider than the filament and is about 45 nm in length. It is made up of protein subunits, which are antigenically different from those of flagellin subunits.

c) Filament

The outermost structure of the flagellum is the filament. The filament is a helical structure with a 14 nm diameter. It is composed of subunits, or monomers, of a protein known as flagellin.

The molecular weight of the flagellin is about 40,000. The shape of the flagellum is dependent on flagellin.

The flagellum grows at its tip rather than at the base as the newly synthesized flagellin monomers are added at the distal end of the filament.

Hydrodynamics of flagella:

Bacteria propel themselves by rotating their helical flagella. Imagine a corkscrew penetrating a piece of cork to illustrate the principle involved. In the case of bacteria, the cork is analogous to the viscous medium and the corkscrew to the helical flagellum.

The nature of the rotatory motor that spins each corkscrew-shaped flagellum is still not understood, but the rings found in the basal body are probably involved.

It is known that the flagella are derived from the electrical potential and the hydrogen ion gradient across the cytoplasmic membrane.

According to recent studies, the concentration of cGMP within the cell governs the direction in which the rotation occurs.

Swimming motility without flagella

Certain helical bacteria (spirochetes) exhibit swimming motility, particularly in highly viscous media, yet they lack flagella.

Inside the bacteria, flagella-like structures are present, that is, “axial fibril” (or) “endoflagella,” responsible for the motility of the spirochetes; the mechanism is not yet clear.

Gliding motility

Some bacteria (e.g., Cytophage species) are motile only when they are in contact with a solid surface. This type of motion is comparatively slow (a few mm/sec). No special organelles have been observed.

2. Pili (Fimbriae): Bacterial Cell Adhesion Structures

Many negative bacteria possess minute, shorter, hair-like hollow appendages called the “pili.”

They originate from the cell wall. They are composed of protein and pillin, and they are organs of attachment.

The one kind of pilus called a sex pilus serves as part of the entry of DNA during bacterial conjugation.

Some pili play a major role in human infection by allowing pathogenic bacteria to attach to epithelial cells lining the respiratory, intestinal, or genitourinary tracts.

3. Capsule: Bacterial Cell Protective Layer

A viscous layer called the slime layer (or capsule) surrounds some bacterial cells. The capsule is secreted by the cytoplasm and contains polysaccharides or peptides. It protects the bacteria and may serve as a reservoir of stored food. Or waste products. Capsulated bacteria are infectious, while non-capsulated bacteria are noninfectious.

The main functions of the capsule or slime layer are

1. It may protect against temporary drying by binding water molecules
2. The bacterial cell wall is a critical structural component that defines bacterial cell morphology and provides protection. Understanding the differences between gram-positive and gram-negative bacterial cell walls is essential for studying bacterial cell structure and function. The cell wall composition, primarily consisting of peptidoglycan, varies between bacterial species and plays a crucial role in bacterial identification and antibiotic targeting.
3. They may block the attachment of bacteriophages.
4. They may promote attachment of bacteria to surfaces.

Most bacterial capsules are composed of polysaccharides; few are polypeptides.

Capsules composed of a single kind of sugar are termed homopolysaccharides; those with several kinds of sugars are termed heteropolysaccharides.

Bacterial Cell Wall: Structure and Composition

The bacterial cell wall is surrounded by a rigid cell wall.

The cell wall is one of the most important layers of prokaryotic cells.

In bacteria generally, the layer or layers of the cell envelope lying between the inner cytoplasmic membrane and the capsule are called the “cell wall.”

The main functions of the cell wall are:

1. It provides shape and rigidity to the cell.
2. It provides solid anchoring support to flagella.
3. It prevents the lysis or rupture of bacteria due to osmotic pressure.
4. It contributes to the pathogenicity of many pathogens.
5. It can protect a cell from toxic substances and is the site of action for several antibiotics.

The “cell wall,” a very rigid structure that gives shape to the cell, is located beneath external structures such as capsules, sheaths, and flagella, and outside the cytoplasmic membrane.

Its main function is to prevent the cell from expanding and eventually bursting because of the uptake of water, since most bacteria live in a hypotonic environment.

The walls of gram-negative species are generally thinner (10 to 15 nm) than those of gram-positive species (20 to 25 nm). The walls of gram-negative archaebacteria are also thinner than those of gram-positive archaebacteria.

Chemical composition

For eubacteria, the shape-determining part of the cell wall is large “peptidoglycan” (sometimes called “murein”), an insoluble, porous, cross-linked polymer of enormous strength and rigidity.

The peptidoglycan is found only in prokaryotes; it occurs in the form of a “bag-shaped macromolecule” surrounding the cytoplasmic membrane.

The peptidoglycan is essentially composed of the following polymers:

• N-Acetylglucosamine,
• N-Acetylmuramic acid,
• L-Alanine,
• D-Alanine,
• D-Glutamic acid and
• Diamino acid

(LL – (or) meso = diamino pimelic acid, L-Lysin, L-Ornithine, (or) L-diamino butyric acid

Archaebacteria possess cell walls, which do not contain peptidoglycan, and their cell wall fine structure and chemical composition are very different from those of eubacteria.

Their cell walls are usually composed of proteins, glycoproteins, or polysaccharides.

• Methanobacterium has cell walls—pseudomurein.

Bacteria are divided into 2 types, namely gram-positive and gram-negative, based on the differences in the reaction to Gram’s staining.

Gram-Positive Bacterial Cell Wall Structure

Gram-positive bacteria usually have a much greater amount of peptidoglycan in their cell walls than do gram-negative bacteria (~50% or more of the dry weight), but only in about 10% of the wall of gram-negative bacteria.

Other substances may occur in addition to peptidoglycan.

1. Streptococcus pyogenes contains “polysaccharides,” which can be extracted with hot-dilute HCl.
2. “Streptococcus aureus” and “Streptococcus faecalis” contain “teichoic acid”—acidic polymers of ribitol phosphate (or) glycerol phosphate, which can be extracted with cold dilute acid.
3. Teichoic acids bind Mg²⁺ ions, and there is some evidence that they help to protect bacteria from thermal injury by providing an accessible pool of these cations for stabilization of the cytoplasmic membrane.
4. The walls of the bacteria can be destroyed by treatment with an enzyme called “lysozyme,” which selectively dissolves peptidoglycan.

Gram-Negative Bacterial Cell Wall Structure

The walls of Gram-negative bacteria are more complex than those of Gram-positive bacteria.

The most interesting difference is the presence of an outer membrane’ that surrounds a thin underlying layer of peptidoglycan.

The walls of Gram-negative bacteria are rich in lipids (11–22% dry weight of the wall).

• This outer membrane serves as an impermeable barrier to prevent the escape of important enzymes, such as those involved in cell wall growth, from the space between the cytoplasmic
• Gram-negative bacteria are refractory to the enzyme “lysozyme” (or) resistant to the lysosomal treatment).
• Only if the outer membrane is first damaged, as by removal of stabilizing “magnesium ions” by a chelating agent, can the enzyme penetrate and attack the underlying peptidoglycan layer?
• The outer membrane of the gram-negative cell wall is anchored to the underlying peptidoglycan by means of “Braun’s lipopolysaccharides.”
• The membrane is a bi-layered structure consisting mainly of “phospholipids, proteins, and lipopolysaccharides (LPS).”
• The LPS has toxic properties and is also known as “endotoxin.” It is composed of three covalently linked parts.
  • a) Lipid. A: Firmly embedded in the membrane.
  • b) Core polysaccharide: Located at the membrane surface.
  • c) O-linked polysaccharide: which extends like whiskers from the membrane surface into the surrounding medium. Many of the serological properties of Gram-negative bacteria are attributable to O-antigens; they can also serve as receptors for bacteriophage attachment.

Internal Bacterial Cell Structures and Components

The structures that are present interior to the cell wall include the cell membrane, mesosomes, cytoplasm, nuclear material, cytoplasmic inclusions, and vacuoles.

i. The cell membrane (or) Cytoplasmic membrane

Immediately beneath the cell wall, there is a thin membrane around the cytoplasm.

This membrane is called the cytoplasmic membrane or plasma membrane. It contains phospholipids, proteins, and polysaccharides.

Functions:

• selective permeability and transport of solutes.
• Electron transport and oxidative phosphorylation in aerobic species.
• Excretion of hydrolytic exoenzymes.
• Bacteria bear the enzymes and carrier molecules that function in the biosynthesis of DNA, cell wall polymer, and membrane lipids.
• These structures bear the receptors and other proteins involved in chemotactic and sensory transduction systems.

ii. Mesosomes

In gram-positive bacteria, the plasma membrane forms by folding inward.

The infolding can give rise to mesosomes within the cytoplasm.

The mesosome is associated with bacterial nuclear material and its replication. Respiratory enzymes are also associated with thin mesosomes.

Functions:

• They are involved in cell wall formation during cell division.
• They play a role in the replication of chromosomes and distribution to daughter cells.
• They are also involved in secretory processes.

iii. Cytoplasm

The cytoplasm is an aqueous solution of soluble proteins and metabolites.

It contains RNA, ribosomes, and reserve food materials. It also contains a nuclear body, or nucleoid.

iv. Nuclear material

The prokaryotic cell is strikingly different from the eukaryotic cell in the lack of a well-defined nucleus.

The plasmids are non-chromosomal genetic material. Plasmids are circular, double-stranded DNA molecules with independent replicating capacity; they are smaller in size.

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v. Ribosomes

In bacterial cells, ribosomes occur freely in the cytoplasm and in clusters of 4–6 ribosomes. The clusters are called the polysomes.

The ribosome is of the 70S type, consisting of 50S and 80S particles. The cytoplasmic area, granular in appearance and rich in ribosomes, is where protein is synthesized.

vi. Cytoplasmic inclusions and Vacuoles

Concentrated deposits of certain substances are detectable in the cytoplasm of some bacteria. The reserved food materials are fats, polysaccharides, volutin, etc.

Volutin granules, also known as metachromatic granules, are composed of polyphosphate. Volutin serves as a reserve source of phosphate.

Fats are stored in highly retractile globules in the form of polymerized β-hydroxybutyric acid, which can serve as a reserve carbon and energy source. Polysaccharides (glycogen) are stored in granules.

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Another type of inclusion is represented by the intracellular globules of elemental sulfur that may accumulate in certain bacteria growing in environments rich in hydrogen sulfide.

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Some bacteria that live in aquatic habitats form gas vacuoles that provide buoyancy. These gas vacuoles are bright retractile bodies that have a regular shape, resembling hollow, rigid cylinders with more or less conical ends, and possess a striated protein boundary. This boundary is impermeable to water.