6.4 Structure, Shape and Size of Bacteria
This section details the fundamental structural components of a typical bacterium, including its cell wall, external features, and genetic material. It explores the diversity in bacterial shape, size, and motility, with a focus on key survival mechanisms like endospore formation.
Prokaryotic and Eukaryotic Cells→
General Bacterial Structure
A typical bacterium comprises several essential components:
- Cell Wall: A rigid outer layer that provides shape and structural support. (Mycoplasma species are an exception, lacking a cell wall).
- Cell Membrane: Located inside the cell wall, it regulates the passage of substances.
- Cytoplasm: The gel-like substance filling the cell.
- Genetic Material: Typically a single, circular DNA molecule located in the nucleoid region.
- External Structures: Some bacteria possess additional features outside the cell wall, such as a capsule, flagella, and pili.
The Bacterial Cell Wall
The cell wall is a crucial, multi-layered structure external to the cytoplasmic membrane.
- Peptidoglycan (Murein/Mucopeptide): This is the core component, forming a complex, interwoven network that surrounds the cell. It is found only in bacterial cell walls and is responsible for maintaining the cell's characteristic shape. It is composed of peptides and sugars (glycans).
Cell Wall and Plasma Membrane→
Cell Walls of Gram-Positive vs. Gram-Negative Bacteria
Bacteria are broadly classified into two groups based on their cell wall structure, which can be differentiated using a staining technique called Gram staining.
| Feature | Gram-Positive Bacteria | Gram-Negative Bacteria |
|---|---|---|
| Peptidoglycan Layer | Very thick | Very thin |
| Teichoic Acids | Present, protrude from the peptidoglycan layer | Absent |
| Outer Membrane | Absent | Present (composed of lipopolysaccharides, lipoproteins, phospholipids) |
| Periplasmic Space | Absent | Present (space between outer and cytoplasmic membranes) |
| β-lactamases | N/A | Can be present in the periplasmic space, providing antibiotic resistance |
External Structures
- Capsule: A gelatinous, polysaccharide layer covering the bacterium (exception: Bacillus anthracis has a polypeptide capsule). It can aid in adherence to host tissues.
Diversity of Bacterial Shapes and Sizes
- Size: Bacteria range from 0.1 to 600 µm.
- Shapes: There are three primary shapes with various arrangements.
| Main Shape | Description | Arrangements and Examples |
|---|---|---|
| Spherical (Coccus) | Round cells. | Diplococci: in pairs (e.g., Neisseria meningitidis) Streptococci: in chains (e.g., Streptococcus pneumoniae) Staphylococci: in grape-like clumps Tetrad: packets of 4 cells Sarcina: packets of 8 cells |
| Rod-shaped (Bacillus) | Elongated, rod-like cells. | Diplobacillus: in pairs Coccobacilli: short, ovoid rods Vibrio: curved, comma-shaped Palisade: stacked like coins (e.g., Escherichia coli, Pseudomonas) |
| Spiral | Helical or corkscrew-shaped cells. | Spirochetes: thin, flexible spirals that occur singly (e.g., Treponema pallidum) |
Endospore Formation (Sporulation)
This is a survival mechanism, not a form of reproduction, used by bacteria like Clostridium and Bacillus to withstand harsh environmental conditions.
- Process: A single bacterium forms a single, dormant, and highly resistant endospore.
- Composition: The spore contains DNA, a small amount of cytoplasm, ribosomes, peptidoglycan, very little water, and a thick, keratin-like coat.
- Stages of Sporulation:
- Initiation: Triggered by unfavorable conditions; DNA replicates.
- Asymmetric Division: The cell divides into a large mother cell and a smaller forespore.
- Membrane Development: Protective membranes form around the forespore.
- Peptidoglycan Layer: The mother cell adds peptidoglycan layers to the forespore's core.
- Spore Coat Formation: A protective protein coat is added.
- Maturation: The endospore dehydrates.
- Release: The mother cell lyses, releasing the mature endospore.
- Germination: When conditions become favorable, the endospore germinates back into an active (vegetative) cell.
Motility in Bacteria
- Mechanisms:
- Flagella: Thread-like outgrowths used for swimming.
- Twitching Motility: Pili are used as grappling hooks.
- Bacterial Gliding: Movement over surfaces without flagella.
- Changes of Buoyancy: For movement in aquatic environments.
- Taxis: Directed movement in response to a stimulus.
- Chemotaxis: Response to chemicals.
- Phototaxis: Response to light.
- Magnetotaxis: Response to magnetic fields.
- Flagellar Arrangements:
- Monotrichous: A single flagellum.
- Amphitrichous: A flagellum at each end.
- Lophotrichous: A cluster of flagella at one or both poles.
- Peritrichous: Flagella distributed over the entire cell surface.
Structure of a Bacterial Flagellum
The bacterial flagellum is a simple yet powerful motor.
- Composition: Made of a protein called flagellin; lacks microtubules.
- Parts:
- Basal Body: Anchors the flagellum in the cell wall and membrane and produces a 360° rotatory motion.
- Hook: A flexible coupling that connects the basal body to the filament.
- Filament: A long, hollow, helical structure that acts as the propeller.
Genomic Organization
- Chromosome: Most bacteria have a single, circular DNA chromosome. The size varies greatly between species (e.g., from 160,000 to 12.2 million base pairs). Exception: Borrelia species have a linear chromosome.
- Plasmids: Many bacteria contain small, accessory rings of DNA called plasmids. These are separate from the main chromosome and often carry genes for antibiotic resistance or other special functions.
- Mesosomes: These are internal extensions of the cell membrane into the cytoplasm. They are involved in DNA replication, cell division, and export of exocellular enzymes.