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Bacterial Cell Structure

For medical students2 min readUpdated 2026-10-10

A bacterial cell is a complex microscopic system consisting of surface layers, internal protoplasmic contents, and various non-essential elements. A detailed understanding of its ultrastructure is essential for studying physiological processes, mechanisms of division, and microorganism-environment interactions.

Study methodElectron microscopy (whole cells and ultrathin sections)

Main threatHigh internal osmotic pressure

Wall thicknessGram-positive — from 50 nm, Gram-negative — 15–20 nm

Genetic apparatusNucleoid (nuclear equivalent), plasmids, and mRNA

General Architecture and Research Methods

To study microorganism morphology in detail, electron microscopy is primarily used. This method allows the visualization of both whole cells and their ultrathin sections, making it possible to observe the finest structural details.

All major structural elements of a bacterium can be divided into three large groups:

Envelope, which contacts the external environment and protects the cell.
Internal contents (protoplasm), where the main metabolic and life cycles take place.
Additional structures, which are not present in all species, but provide specific survival or motility advantages.

Cell Envelope: Wall and Membrane

The bacterial surface apparatus includes several layers. Directly beneath the outer structures lies the cell wall—a rigid and elastic structure that gives the microorganism its characteristic shape. Beneath the cell wall is the cytoplasmic membrane.

Functionally, the cell wall acts as a containment barrier. It provides vital counter-pressure against high internal osmotic pressure, preventing the bacterium from lysing. In addition, the wall participates in key physiological processes:

Cell division.
Transport of various metabolites.
Receptor functions (bearing specific receptors for bacteriophages, bacteriocins, and various chemicals on its surface).

An important diagnostic feature is the comparative thickness of the cell wall. In Gram-negative bacteria, it is relatively thin, measuring about 15–20 nm. Conversely, in Gram-positive bacteria, the wall is much thicker and can reach 50 nm or more.

Protoplasmic Ultrastructure and Genetic Apparatus

The interior space of a bacterium is filled with cytoplasm. When examining ultrathin sections, various functional elements are revealed within the cytoplasm:

Ribosomes — the protein synthesis machinery.
Inclusions — for example, volutin granules, serving as nutrient reserves.
Mesosomes — intracellular membrane invaginations.

The genetic material of a bacterium is not enclosed in a true nucleus like that of eukaryotes. Its function is performed by the nucleoid (nuclear equivalent). In addition to the main nucleoid, the genetic apparatus includes extrachromosomal elements (plasmids) and messenger RNA (mRNA) molecules.

Additional Bacterial Cell Structures

Many bacteria possess non-essential structures, the presence of which depends on the microbial species and environmental conditions. Protective surface layers may include a capsule, microcapsule, or slime layer, establishing an additional barrier against external stressors.

For locomotion and surface attachment, specialized movement and adhesion organelles are utilized: flagella and pili. Of particular importance are spores—specialized dormant forms that bacteria form to maintain viability under extremely adverse environmental conditions.

Mnemonic

To remember the envelope layers from outside in, use the phrase "Capsule Stays Central": Capsule (optional) → Cell Wall → Cytoplasmic Membrane.

Frequently asked questions

How do the cell walls of Gram-positive and Gram-negative bacteria differ?

The cell walls of Gram-positive and Gram-negative bacteria differ in thickness, chemical composition, and cross-linking.

Feature	Gram-positive bacteria	Gram-negative bacteria
Thickness	Up to 50 nm or more	Thin (approx. 15–20 nm)
Peptidoglycan	Multilayered (40–90% of dry mass)	Thin layer (5–10% of dry mass)
Specific components	Teichoic and lipoteichoic acids	Outer membrane with lipopolysaccharide (LPS), porins, and lipoproteins
Interpeptide cross-links	Via pentaglycine bridge	Direct peptide bond
Peptidoglycan localization	Tightly adherent to membrane	In the periplasmic space

Which bacterial cell structures are classified as essential?

Essential bacterial cell structures include elements of the envelope and the internal contents (protoplasm).

Cell wall — a rigid, elastic structure that gives the cell its shape.
Cytoplasmic membrane — located directly beneath the cell wall.
Cytoplasm — the internal matrix containing soluble proteins, RNA, ribosomes, and inclusions.
Nucleoid — the genetic apparatus (nuclear equivalent).

Capsules, flagella, pili, and spores are additional (non-essential) structures.

What types of inclusions are found in the bacterial cytoplasm?

Inclusions found in the bacterial cytoplasm primarily serve as nutrient reserves. The main types include:

Volutin granules — specific cytoplasmic storage granules (metachromatic granules).
Lipids — fat droplets serving as reserve nutrients.

These structures reside in the cytoplasm alongside soluble proteins, RNA, and ribosomes.

What functions do mesosomes perform in a bacterial cell?

Mesosomes participate in energy-demanding processes and energy metabolism within the bacterial cell. Their functions include:

Energy generation — supporting cellular energy requirements.
Cell division — participating in DNA segregation and growth.
Biosynthesis — providing energy for cell wall, carbohydrate, and lipid synthesis.
Secretion — exporting compounds out of the cell.
Sporulation — participating in spore formation.

What is the chemical composition of the bacterial capsule?

The bacterial capsule is predominantly composed of polysaccharides (exopolysaccharides). Polypeptide capsules are rare exceptions; for instance, in the anthrax bacillus (Bacillus anthracis), the capsule consists of polymers of D-glutamic acid. The capsule exhibits marked hydrophilicity, enabling the structure to retain large amounts of water.

What functions does the bacterial cytoplasmic membrane perform?

The bacterial cytoplasmic membrane performs the following functions:

Regulation of osmotic pressure.
Transport of substances via permeases.
Energy metabolism.
Cell growth and division.
Motility (via anchoring flagellar motors).
Secretion.
Sporulation.

Which structure protects the bacterium from bursting due to high internal pressure?

The cell wall performs this function. It possesses high strength and elasticity, counteracting high internal osmotic pressure.

How do the cell walls of Gram-positive and Gram-negative bacteria differ?

The primary difference visible via microscopy is thickness. Gram-positive bacteria have a thick cell wall (50 nm or more), whereas Gram-negative bacteria have a thin wall (15–20 nm).

Where is bacterial genetic information stored?

In the nucleoid, which serves as the nuclear equivalent since bacteria lack a true nucleus. Genetic information can also be housed in plasmids.

Why do bacteria form spores?

Spores are not meant for reproduction. They are a specialized form for preserving genetic material and surviving harsh environmental conditions.

Go deeper

Mechanisms of metabolite transport across the cell wall
Specificity of receptors for bacteriophages and bacteriocins
Role of mesosomes and volutin inclusions in the cytoplasm
Structure and functions of plasmids in the bacterial cell
Differences between the capsule, microcapsule, and slime layer