Bacterial Cell Structure

Introduction

Bacteria are prokaryotic microorganisms with a relatively simple internal structure compared to eukaryotes. Their cellular organization is adapted for survival in diverse environments. Understanding bacterial cell structure is fundamental to microbiology, as it relates to bacterial physiology, pathogenicity, and response to antimicrobial agents.

Major Bacterial Shapes

• Coccus (Cocci): Spherical or round-shaped bacteria.

• Bacillus (Bacilli): Rod-shaped bacteria.

• Spirillum (Spirilla): Spiral-shaped bacteria.

• Vibrio: Comma-shaped bacteria.

• Spirochete: Corkscrew-shaped bacteria.

• Stellate: Star-shaped (rare; found in some Archaebacteria).

• Square: Square-shaped (rare; found in some Archaebacteria).

Structures External to the Cell Wall

Flagella

• Structure: Composed of flagellin protein subunits forming three strands twisted into a helix. The flagellum consists of the filament, hook, and basal body (anchored by rings in the cell wall/membrane; arrangement differs in Gram-positive and Gram-negative bacteria).

• Function: Provides motility and chemotaxis (movement toward or away from chemical stimuli) via runs and tumbles.

• Arrangements:

  • Monotrichous: Single flagellum at one end

  • Lophotrichous: Cluster of flagella at one or both ends

  • Amphitrichous: Single flagellum at both ends

  • Peritrichous: Flagella distributed over the entire cell surface

• Detection Methods: Electron microscopy, flagella staining, hanging drop motility test, and stab technique in semisolid medium.

Pili (Fimbriae)

• Structure: Hair-like projections made of pilin protein; can be long or short, hollow tubes.

• Types:

  • Common pili: Short, numerous, chromosome-encoded; function in adherence (e.g., Neisseria gonorrhoeae), pellicle formation, and as bacteriophage invasion sites.

  • F pilus (sex pilus): Long, usually single, plasmid-encoded; mediates conjugation (transfer of genetic material between cells).

Glycocalyx (Extramural Layers)

• Capsule: Thick, organized polysaccharide or glycoprotein layer produced by young cells in carbohydrate-rich environments (e.g., Streptococcus mutans).

• Functions of Capsule:

  • Protection from phagocytosis (e.g., Streptococcus pneumoniae, Bacillus anthracis).

  • Virulence factor (essential for disease in some bacteria).

  • Antigenic properties (basis for some vaccines, e.g., HIB, Menomune).

  • Adhesion to surfaces (e.g., dental plaque formation by S. mutans).

  • Reservoir for nutrients or waste disposal.

  • Prevents desiccation (important for soil bacteria).

• Slime Layer: Loosely organized, less defined than a capsule; problematic in industrial settings (e.g., dairy, paper industries).

Cell Wall

General Structure and Function

• Function: Provides rigidity, maintains shape, and protects against osmotic lysis.

• Composition: Primarily peptidoglycan (murein), unique to prokaryotes.

• Peptidoglycan Structure:

  • Repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β 1-4 glycosidic bonds.

  • Amino acid chains (tetra- or pentapeptides) attached to NAM; cross-linked by peptide bridges (involving DAP or L-lysine).

Equation for Peptidoglycan Linkage:

Gram-Positive vs. Gram-Negative Cell Walls

Additional Features

• Gram-Positive: May contain additional proteins (e.g., A protein in Staphylococcus aureus, M protein in Streptococcus pyogenes) that enhance virulence.

• Gram-Negative: Outer membrane contains porins (protein channels for nutrient uptake and as receptor sites for bacteriophages/bacteriocins). LPS layer (endotoxin) can cause fever, shock, and other symptoms when released.

Cell Wall Variants

• Mycoplasmas: Lack a cell wall; resistant to cell wall-targeting antibiotics.

• L forms: Mutants with defective or absent cell walls.

• Protoplasts & Spheroplasts: Cells with cell walls removed by enzymes or antibiotics.

Antimicrobial Action on Cell Wall

• Penicillins & Cephalosporins: Inhibit peptide bridge formation (transpeptidation), especially effective against Gram-positive bacteria.

• Lysozyme: Hydrolyzes β 1-4 glycosidic bonds between NAM and NAG.

Structures Internal to the Cell Wall

Cell (Cytoplasmic/Plasma) Membrane

• Composition: Phospholipid bilayer with embedded proteins (fluid-mosaic model).

• Function: Selectively permeable barrier regulating molecular traffic into and out of the cell.

Intracytoplasmic Membranes

• Vesicles: Parallel stacks involved in cellular respiration (ATP production) or photosynthesis (in photosynthetic bacteria).

• Mesosomes: Once thought to be involved in cell division; now considered artifacts of cell preparation.

Ribosomes

• Structure: Composed of RNA and protein; two subunits (50S and 30S in prokaryotes; 60S and 40S in eukaryotes).

• Function: Site of protein synthesis; target for several antibiotics.

Endospores

• Produced by: Mainly Bacillus (aerobic) and Clostridium (anaerobic) species.

• Function: Dormant, highly resistant structures for survival under adverse conditions (heat, desiccation, chemicals, radiation).

• Resistance Mechanism: Due to dipicolinic acid (DPA)-calcium complex in spore coats.

• Detection: Appear as refractile bodies; stained by Shaeffer-Fulton or Dorner methods.

• Sporogenesis: Genetically controlled; not always triggered by unfavorable conditions. Spores germinate into vegetative cells when conditions improve.

Nucleoid

• Structure: Single, circular, double-stranded DNA molecule (not membrane-bound).

• Function: Contains genetic information (~3,000 genes); acts as the control center of the cell.

Plasmids

• Structure: Small, circular, extrachromosomal DNA molecules.

• Function: Carry genes for traits such as antibiotic resistance; can be transferred between cells via conjugation; important tools in biotechnology (as vectors for gene cloning).

Cell Inclusions (Granules)

• Metachromatic (volutin) granules: Polymetaphosphate storage; characteristic of Clostridium.

• Fat inclusions: Lipid granules (β-hydroxybutyric acid).

• Polysaccharide granules: Energy reserves.

• Iron or sulfur granules: Found in thermophilic bacteria (e.g., in hot springs).

• Magnetosomes: Magnetite granules for orientation in geomagnetic fields; found in strict anaerobes.

• Gas vesicles: Provide buoyancy in photosynthetic bacteria and cyanobacteria.

Summary Table: Key Bacterial Cell Structures

Additional info:

• Some details about the function of teichoic acids, porins, and the role of LPS in disease were expanded for clarity.

• Examples of bacteria and clinical relevance (e.g., vaccine development, antibiotic resistance) were added for context.