Structure and chemical composition of bacterial cell Wall

Structure and chemical composition of cell wall, cytoplasm membrane, protoplasts, spheroplasts the cytoplasm, nuclear material. The bacterial cell wall is a rigid structure surrounding the cytoplasmic membrane that gives the cell its shape, mechanical strength, and protection against osmotic pressure.
It is the primary feature distinguishing Gram-positive and Gram-negative bacteria.

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B. Functions

• Maintains cell shape and rigidity
• Prevents osmotic lysis
• Anchors appendages like flagella
• Provides antigenic properties (e.g., O antigen)
• Acts as a site of action for many antibiotics (e.g., penicillin targets peptidoglycan)

C. Structure and Chemical Composition

The major chemical component of the bacterial cell wall is peptidoglycan (murein) — a unique polymer found only in bacteria.

Peptidoglycan Composition:

• Polysaccharide backbone: Alternating units of
  • N-acetylglucosamine (NAG)
  • N-acetylmuramic acid (NAM)
• Each NAM has a short tetrapeptide side chain (L-alanine, D-glutamic acid, meso-diaminopimelic acid, D-alanine).
• Peptide chains are cross-linked by peptide bridges, forming a strong, mesh-like network.

D. Gram-positive vs. Gram-negative Cell Walls

E. Special Types of Cell Walls

• Acid-fast bacteria (e.g., Mycobacterium tuberculosis):
  Cell wall contains mycolic acids, waxes, and arabinogalactan, making it waxy and resistant to stains and chemicals.
• Mycoplasma:
  No cell wall → cell membrane strengthened by sterols (cholesterol-like molecules).

2. Cytoplasmic (Plasma) Membrane

A. Structure

The cytoplasmic membrane is a thin (7–8 nm), semi-permeable layer located beneath the cell wall.
It follows the Fluid Mosaic Model — composed of:

• Phospholipid bilayer (40%)
• Proteins (60%) — integral and peripheral

Phospholipids:
Contain hydrophilic (polar) heads and hydrophobic (non-polar) tails, forming a bilayer.
Proteins:
Act as enzymes, transport channels, and receptors.

B. Chemical Composition

• Lipids: Phosphatidylethanolamine, phosphatidylglycerol, cardiolipin
• Proteins: Enzymatic and structural proteins
• Carbohydrates: Very small amounts (in glycoproteins or glycolipids)

C. Functions

• Selective permeability – regulates entry/exit of substances
• Respiration and energy production – contains enzymes for the electron transport chain (no mitochondria in bacteria)
• Biosynthesis of lipids and peptidoglycan
• Transport of nutrients and ions (via active/passive mechanisms)
• Anchoring site for DNA and flagella
• Secretion of enzymes and toxins

3. Protoplasts and Spheroplasts

These are wall-deficient bacterial forms, produced by lysozyme action or antibiotic treatment.

A. Protoplast

• Derived from Gram-positive bacteria after complete removal of cell wall.
• Surrounded only by the cytoplasmic membrane.
• Spherical, osmotically fragile, can survive only in isotonic solutions.

Formation:

Cell wall destroyed by lysozyme, which hydrolyzes the β-1,4 linkages between NAG and NAM in peptidoglycan.

Example:
Bacillus subtilis → forms protoplasts after lysozyme treatment.

B. Spheroplast

• Derived from Gram-negative bacteria after partial removal of the cell wall.
• Retains outer membrane remnants in addition to cytoplasmic membrane.
• Less fragile than protoplasts.

Example:
E. coli or Salmonella typhi → form spheroplasts upon penicillin or lysozyme exposure.

C. Importance

• Useful in cell wall biosynthesis studies.
• Serve as host systems for DNA uptake (transfection).
• Basis for L-forms (bacteria that can grow without cell walls under certain conditions).

4. Cytoplasm

A. Structure

The cytoplasm is a viscous, semi-transparent, gel-like matrix enclosed by the plasma membrane.
It lacks membrane-bound organelles but contains macromolecules, inclusions, and ribosomes.

B. Chemical Composition

• Water: 80–90%
• Proteins: Enzymes, structural proteins
• Carbohydrates: Energy source
• Lipids: Membrane synthesis
• Inorganic ions: K⁺, Mg²⁺, phosphate, sulfate
• Nucleic acids and ribosomes

C. Components of Cytoplasm

1. Ribosomes (70S):
   1. Site of protein synthesis
   1. Made of rRNA (23S, 16S, 5S) and proteins
   1. Target for antibiotics like tetracycline and erythromycin
2. Inclusion bodies:
   1. Reserve materials such as glycogen, sulfur, polyphosphate, or poly-β-hydroxybutyrate
   1. Example: Corynebacterium diphtheriae → metachromatic granules (volutin)
3. Gas vacuoles:
   1. In aquatic bacteria and cyanobacteria; control buoyancy.
4. Magnetosomes:
   1. Contain magnetite (Fe₃O₄) — orient bacteria along magnetic fields.

D. Functions of Cytoplasm

• Site for metabolic reactions (glycolysis, biosynthesis)
• Houses genetic material and ribosomes
• Maintains internal environment and turgor pressure

5. Nuclear Material (Nucleoid)

A. Structure

• The bacterial nucleus is not enclosed by a nuclear membrane; hence, called the nucleoid.
• Contains the bacterial chromosome, a single, circular, double-stranded DNA molecule.

Size:

• ~1 mm long when uncoiled, with 4 × 10⁶ base pairs in E. coli.
• Supercoiled and attached to the plasma membrane at one point (the origin of replication).

B. Chemical Composition

• DNA: Double-stranded helix, containing genes for cell structure and function.
• RNA: mRNA, rRNA, tRNA synthesized from DNA templates.
• Proteins: Enzymes for replication, transcription, and DNA packaging.

C. Plasmids (Extra-chromosomal DNA)

• Small, circular DNA molecules independent of chromosomal DNA.
• Carry non-essential genes (e.g., antibiotic resistance, virulence factors).
• Capable of replicating autonomously.
• Transferred during bacterial conjugation via sex pili.

D. Functions of Nucleoid and Plasmids

• Storage of genetic information
• Replication and gene expression
• Adaptation and evolution via horizontal gene transfer

6. Table