Morphology and fine structure of bacteria

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Morphology and fine structure of bacteria are microscopic, single-celled prokaryotic organisms that lack a true nucleus and membrane-bound organelles.
They are among the earliest and simplest forms of life, yet they exhibit remarkable structural diversity and metabolic versatility.

Understanding the morphology (shape and size) and fine structure (internal and external components) of bacteria is essential for studying their classification, identification, and pathogenicity.

2. Morphology of Bacteria

A. Size of Bacteria

Bacteria are extremely small, typically measuring between 0.2 µm to 5 µm in length.

• Average size: 1–2 µm × 0.5 µm (for common rod-shaped bacteria like E. coli)
• Smallest bacteria: Mycoplasma (~0.2 µm, near viral size)
• Largest bacteria: Thiomargarita namibiensis (~750 µm in diameter)

B. Shapes of Bacteria

Bacterial morphology is mainly determined by the cell wall and cytoskeleton-like proteins.

C. Arrangements of Bacteria

Cocci and bacilli often exhibit characteristic arrangements due to the pattern of cell division:

3. Fine Structure of Bacteria (Cell Components)

The bacterial cell is structurally simpler than a eukaryotic cell, yet it is highly organized.
It consists of three main parts:

A. Cell envelope (external structures)
B. Cytoplasmic region (internal structures)
C. Surface appendages (flagella, pili, fimbriae)

A. Cell Envelope

The cell envelope includes all layers surrounding the cytoplasm: cell wall, plasma membrane, and sometimes a capsule.

1. Capsule (Glycocalyx)

• Outermost, gelatinous layer composed of polysaccharides or polypeptides.
• Provides protection against desiccation, phagocytosis, and immune response.
• May appear as:
  • Capsule (organized, firmly attached)
  • Slime layer (loose, unorganized)
• Example: Klebsiella pneumoniae, Streptococcus pneumoniae.

2. Cell Wall

• A rigid structure that maintains cell shape, prevents osmotic lysis, and gives structural support.
• Composed mainly of peptidoglycan (murein) — a complex of sugars and amino acids.

Structure of Peptidoglycan:

• Alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).
• Linked by short tetrapeptide side chains forming a 3D mesh network.

3. Differences Between Gram-positive and Gram-negative Cell Walls

Lipopolysaccharide (LPS) in Gram-negative bacteria acts as endotoxin, contributing to fever and shock during infection.

4. Plasma (Cytoplasmic) Membrane

• A phospholipid bilayer with embedded proteins (about 60% protein, 40% lipid).
• Functions as a selective barrier for transport, respiration, and biosynthesis.

Functions:

• Energy generation (ATP synthesis via electron transport chain)
• Nutrient and ion transport
• Synthesis of cell wall precursors and lipids
• Secretion of enzymes and toxins

B. Cytoplasmic Region

1. Cytoplasm

• Gel-like matrix containing enzymes, nutrients, ribosomes, DNA, and inclusion bodies.
• Site of metabolic reactions.

2. Nucleoid

• Region containing the bacterial chromosome (single, circular, double-stranded DNA molecule).
• Not enclosed by a membrane.
• Contains about 2000–5000 genes.
• DNA is supercoiled and attached to the cell membrane at one point.

3. Plasmids

• Small, circular extra-chromosomal DNA molecules.
• Carry genes for antibiotic resistance, toxin production, or conjugation (F plasmid).
• Replicate independently of the main chromosome.

4. Ribosomes

• Sites of protein synthesis.
• Bacterial ribosomes are 70S, made of 50S and 30S subunits.
• Contain rRNA and proteins.
• Target for many antibiotics (e.g., tetracycline, erythromycin).

5. Inclusion Bodies (Storage Granules)

Store nutrients or metabolic by-products:

C. Surface Appendages

1. Flagella

• Long, thread-like structures responsible for motility.
• Composed of flagellin protein.
• Rotates like a propeller (not whip-like motion).

Structure:

• Filament: Long, helical part extending outside.
• Hook: Connects filament to basal body.
• Basal body: Anchors flagellum to cell wall and membrane; motor function.

Flagellar Arrangements:

2. Pili (Fimbriae)

• Short, hair-like projections made of pilin protein.
• Found mainly in Gram-negative bacteria.

Types:

• Fimbriae: Numerous, short; help in adhesion to host tissues or surfaces (important in pathogenicity).
• Sex pili: Longer, fewer; involved in DNA transfer (conjugation).
• D. Endospores
• Highly resistant, dormant structures formed by certain Gram-positive bacteria (e.g., Bacillus, Clostridium).
• Formed under unfavorable conditions.

Characteristics:

• Resistant to heat, radiation, desiccation, and chemicals.
• Contains dipicolinic acid and calcium ions for protection.
• Germinate into vegetative cells under favorable conditions.

4.Table