Size, shape and arrangement of bacterial cells, Flagella and motility, pili, capsule, sheaths, prosthecae and stalks

Size of Bacteria

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Bacteria are microscopic unicellular organisms that vary greatly in size depending on the species and environmental conditions.

• Average bacterial size: 1–5 µm (micrometers) in length and 0.2–2 µm in width.
• Smallest bacteria: Mycoplasma (~0.2 µm) — nearly the size of large viruses.
• Largest bacteria: Thiomargarita namibiensis (~750 µm in diameter).

Examples:

• Escherichia coli → 1.1 × 2 µm
• Streptococcus pneumoniae → 0.8 µm in diameter
• Bacillus anthracis → 1 × 4 µm

Significance:
The small size ensures a high surface area-to-volume ratio, facilitating efficient nutrient uptake, waste removal, and rapid growth.

2. Shape of Bacteria

The shape (morphology) of bacteria is genetically determined and maintained by the cell wall and cytoskeletal proteins.
Different bacterial shapes are often associated with their habitat, mode of reproduction, and pathogenicity.

3. Arrangement of Bacterial Cells

The arrangement depends on the plane of cell division and whether the daughter cells remain attached after division.

4. Flagella and Motility

A. Flagella – Definition and Structure

• Flagella are long, thin, hair-like protein appendages that provide motility to many bacteria.
• Composed of a protein called flagellin and anchored in the cell envelope by a basal body.

Three Main Parts:

1. Filament – Long helical structure extending outside the cell.
2. Hook – Curved segment that connects the filament to the basal body.
3. Basal body – Complex structure that anchors the flagellum and acts as a motor.

Arrangement of Flagella:

B. Mechanism of Motility

• Flagella rotate like a propeller (not whip-like) powered by proton motive force (H⁺ gradient) across the membrane.
• Movement types:
  • Run → flagella rotate counterclockwise → smooth forward movement.
  • Tumble → clockwise rotation → random reorientation.

Chemotaxis:
Movement toward or away from chemical stimuli.

• Attractants: Nutrients (e.g., glucose).
• Repellents: Harmful substances (e.g., phenol).

C. Other Types of Bacterial Motility

• Gliding motility: Slow movement on solid surfaces without flagella (Myxococcus).
• Spirochete motility: Using axial filaments/endoflagella inside periplasmic space (Treponema pallidum).
• Twitching motility: By extension/retraction of type IV pili (Pseudomonas, Neisseria).

5. Pili (Fimbriae)

A. Definition

• Short, thin, hair-like protein appendages (made of pilin) found mainly in Gram-negative bacteria.

B. Types and Functions

Functions:

• Attachment to epithelial cells (important in infection).
• Biofilm formation.
• Gene exchange (horizontal gene transfer).

6. Capsule

A. Definition

• A gelatinous, sticky outer covering made of polysaccharides (sometimes polypeptides).
• Lies outside the cell wall and is tightly bound to the cell.

B. Types

• Capsule: Organized and firmly attached.
• Slime layer: Unorganized and loosely attached.

C. Functions

• Protects against phagocytosis (e.g., Streptococcus pneumoniae).
• Prevents desiccation (drying).
• Aids in attachment to host tissues.
• Helps in virulence (pathogenicity).

Example:
Klebsiella pneumoniae, Bacillus anthracis, Streptococcus pneumoniae.

7. Sheaths

A. Definition

• Tubular or filamentous envelopes that enclose chains of bacterial cells.
• Composed mainly of polysaccharides and protein.

B. Function

• Provides protection against unfavorable environmental conditions.
• Aids in attachment to surfaces (stones, plants, etc.).
• Found in aquatic environments.

Examples:
Leptothrix, Sphaerotilus natans (common in polluted water).

8. Prosthecae and Stalks

A. Prosthecae

• Extension of the cell wall and plasma membrane forming tube-like or finger-like projections.
• Function in attachment, nutrient absorption, and surface colonization.

Example: Caulobacter, Hyphomicrobium.

B. Stalks

• Non-living extensions of the cell envelope with holdfasts (adhesive materials) at the tip.
• Serve mainly for attachment to solid surfaces in aquatic environments.
• Increase surface area for nutrient absorption.

Example: Caulobacter crescentus (notable for its stalk and cell differentiation).

9. Summary Table