---

Structure, Growth, and Classification of Bacteria

---

1. Bacteria vs. Eukaryotes: The Fundamental Distinction

• Single, circular, double-stranded DNA chromosome (~5 million base pairs in E. coli, ~1.3 mm long)
• 70S ribosomes (vs. 80S in eukaryotes) - target for many antibiotics
• Meshlike peptidoglycan cell wall (absent in eukaryotes)
• Size: mostly ~1 µm in diameter (smallest: Chlamydia/Rickettsia at 0.1-0.2 µm)
• Can survive hostile conditions: osmotic extremes, temperature extremes, desiccation

---

2. Bacterial Structure

A. Cytoplasmic Membrane (Plasma Membrane)

• Composition: phospholipids + proteins + enzymes
• Functions: selective permeability, energy generation (electron transport chain), membrane potential, transport of nutrients
• Site of DNA attachment - membrane growth during replication pulls daughter chromosomes apart

B. Cell Wall

Peptidoglycan (Murein, Mucopeptide)

• A rigid meshwork of alternating N-acetylglucosamine (GlcNAc/NAG) and N-acetylmuramic acid (MurNAc/NAM) polysaccharide chains
• Chains are cross-linked by short peptide bridges containing D-amino acids (unusual in nature)
• The diamino acid at position 3 (e.g., lysine, diaminopimelic acid) links to D-alanine at position 4 of an adjacent chain
• In gram-positive bacteria, an amino acid bridge (e.g., pentaglycine in S. aureus) extends the cross-link
• Provides rigidity, determines cell shape, and protects against osmotic lysis
• Lysozyme cleaves the glycan backbone (present in tears, mucus)
• Absent in mycoplasmas; replaced by pseudomurein in Archaea

Gram-Positive Cell Wall

Gram-Negative Cell Wall

Acid-Fast Bacteria (Mycobacteria)

C. External Surface Structures

Capsule

• Polysaccharide layer (some are protein, e.g., B. anthracis poly-D-glutamate)
• Functions: anti-phagocytic (major virulence factor), biofilm formation, immune evasion
• Detected by the Quellung reaction (capsular swelling with specific antibody)
• Examples: Streptococcus pneumoniae, Klebsiella, H. influenzae type b

Flagella

• Protein appendages (flagellin subunits) responsible for motility
• Powered by a proton gradient rotating a basal body motor
• Types by arrangement:
  • Monotrichous - single polar flagellum (e.g., Vibrio cholerae)
  • Lophotrichous - tuft at one pole
  • Amphitrichous - flagella at both poles
  • Peritrichous - flagella all around (e.g., E. coli, Salmonella)
• Basis of H-antigen in serotyping

Pili (Fimbriae)

• Thin protein appendages shorter than flagella
• Common pili: mediate adhesion to host cells (key virulence factor)
• Sex pili (F pili): mediate conjugation (horizontal gene transfer)
• Basis of F-antigen; target for vaccine development (e.g., uropathogenic E. coli)

Spores (Endospores)

• Formed only by gram-positive genera: Bacillus and Clostridium
• Metabolically dormant, highly resistant structures that survive extreme heat, desiccation, UV, disinfectants
• Contain dipicolinic acid (unique spore component) and a thick cortex
• Germinate when conditions become favorable
• Medical importance: B. anthracis (anthrax), C. difficile, C. botulinum, C. tetani

---

3. Bacterial Classification

A. Morphological Classification (Shape)

• Diplococci - pairs (N. gonorrhoeae, S. pneumoniae)
• Streptococci - chains (S. pyogenes)
• Staphylococci - grape-like clusters (S. aureus)
• Tetrads - groups of 4 (Micrococcus)
• Sarcinae - cubical packets of 8

B. Staining Classification

Gram Stain (Most Important)

Acid-Fast Stain (Ziehl-Neelsen)

• Mycobacteria and Nocardia resist decolorization with acid-alcohol
• Appear red against a blue background
• Due to mycolic acid content in cell wall

C. Metabolic/Biochemical Classification

• Oxygen requirements: aerobic, anaerobic, facultative anaerobe, microaerophilic
• Fermentation patterns: lactose fermentation (e.g., E. coli vs. Salmonella)
• Enzyme production: catalase (staphylococci +, streptococci -), coagulase (S. aureus +), oxidase (Pseudomonas +, E. coli -), urease (H. pylori +)
• Hemolysis patterns on blood agar:
  • Alpha (α): partial hemolysis (green) - S. viridans
  • Beta (β): complete hemolysis (clear) - S. pyogenes, S. aureus
  • Gamma (γ): no hemolysis - Enterococcus

D. Antigenic (Serological) Classification

• O-antigen: polysaccharide component of LPS (somatic antigen)
• H-antigen: flagellar protein antigen
• K/Vi-antigen: capsular polysaccharide antigen
• Example: E. coli O157:H7 - the O157 and H7 refer to specific O and H antigens

E. Genetic/Molecular Classification (Modern Gold Standard)

• 16S rRNA gene sequencing - now the standard for definitive species identification
• DNA-DNA hybridization (>70% similarity = same species)
• Linnaean taxonomy: Kingdom → Phylum → Class → Order → Family → Genus → Species
• Family, genus, and species are most clinically useful
• Nomenclature: genus (capitalized, italicized) + species (lowercase, italicized), e.g., Escherichia coli

---

4. Bacterial Growth

Binary Fission

• Bacteria reproduce asexually by binary fission (one cell divides into two identical daughters)
• Process: chromosome replication → elongation → septum formation → cell division
• DNA replication is initiated at the membrane; as membrane grows, daughter chromosomes are pulled apart
• In fast-growing bacteria, new rounds of DNA replication begin before the previous round is complete ("daughters born pregnant")
• Generation time varies widely: E. coli = ~20 minutes; M. tuberculosis = ~20 hours

The Bacterial Growth Curve (Batch Culture)

• Antibiotics targeting cell wall synthesis (beta-lactams, vancomycin) are most effective during the log phase when peptidoglycan synthesis is most active
• Stationary phase bacteria are relatively antibiotic tolerant - relevant in chronic/biofilm infections
• Sporulation is triggered by starvation signals (alarmones) in stationary phase

Environmental Requirements for Growth

---

Quick Reference Summary

BACTERIA
│
├── STRUCTURE
│   ├── Cytoplasmic membrane (energy, transport)
│   ├── Cell wall
│   │   ├── Gram-positive: thick peptidoglycan + teichoic acid
│   │   ├── Gram-negative: thin peptidoglycan + outer membrane + LPS
│   │   └── Acid-fast: mycolic acid (Mycobacteria)
│   └── External appendages
│       ├── Capsule (anti-phagocytic)
│       ├── Flagella (motility)
│       ├── Pili (adhesion, conjugation)
│       └── Spores (Bacillus, Clostridium - dormancy)
│
├── CLASSIFICATION
│   ├── Morphology (cocci, bacilli, spirals)
│   ├── Gram stain (+/-)
│   ├── Acid-fast stain
│   ├── Metabolic properties (oxygen, enzymes)
│   ├── Antigenic (O, H, K antigens)
│   └── Genetic (16S rRNA - gold standard)
│
└── GROWTH
    ├── Binary fission
    ├── Growth curve: Lag → Log → Stationary → Death
    └── Factors: oxygen, temperature, pH, nutrients

---

Structure, Growth, and Classification of Bacteria

(10 Marks)

---

I. Introduction

---

II. Structure of Bacteria

A. Mandatory Structures (Present in All Bacteria)

• Phospholipid bilayer with embedded proteins
• Functions: selective permeability, energy generation, nutrient transport
• Site of DNA attachment during replication

• Peptidoglycan = rigid meshwork of GlcNAc + MurNAc chains cross-linked by peptide bridges
• Cleaved by lysozyme; targeted by beta-lactam antibiotics and vancomycin

• Contains ribosomes (70S = 30S + 50S), plasmids, inclusion bodies, and the nucleoid (circular DNA)

---

B. Optional Structures (Present in Some Bacteria)

• Monotrichous (single polar), Lophotrichous (tuft at one pole), Amphitrichous (both poles), Peritrichous (all around)

---

III. Classification of Bacteria

1. Morphological Classification (Shape & Arrangement)

• Cocci - spherical (Staphylococcus, Streptococcus)
• Bacilli - rod-shaped (E. coli, Bacillus)
• Spiral - Vibrio (comma), Spirillum, Spirochete (Treponema pallidum)
• Coccobacilli - (Haemophilus, Brucella)

• Diplococci (pairs), Streptococci (chains), Staphylococci (clusters), Tetrads (4s), Sarcinae (8s)

2. Staining Classification

• Gram stain - most important; distinguishes gram-positive (purple) from gram-negative (pink)
• Acid-fast stain (Ziehl-Neelsen) - for Mycobacterium and Nocardia (mycolic acid wall)
• Spore stain (Schaeffer-Fulton), Capsule stain (India ink), Flagella stain

3. Metabolic Classification

• Oxygen requirement: Aerobe, Anaerobe, Facultative anaerobe, Microaerophile
• Enzyme tests: Catalase (+: staphylococci / -: streptococci), Coagulase (S. aureus +), Oxidase (Pseudomonas +), Urease (H. pylori +)
• Hemolysis on blood agar:
  • α (partial/green): S. viridans
  • β (complete/clear): S. pyogenes, S. aureus
  • γ (none): Enterococcus

4. Genetic Classification (Modern Gold Standard)

• 16S rRNA gene sequencing - definitive species identification
• Linnaean hierarchy: Kingdom → Phylum → Class → Order → Family → Genus → Species (clinically most useful)
• Nomenclature: Genus species (e.g., Escherichia coli)

---

IV. Growth of Bacteria

Mechanism: Binary Fission

• One cell divides into two identical daughter cells
• Steps: chromosome replication → cell elongation → septum formation → division
• Generation time: E. coli = ~20 min; M. tuberculosis = ~20 hours

The Bacterial Growth Curve

Environmental Factors Affecting Growth

---

V. Clinical Relevance (Value-Adding Points)

• Gram stain guides empirical antibiotic therapy before culture results
• Beta-lactams and vancomycin target peptidoglycan synthesis - active only in log phase
• Spores explain persistence of C. difficile and B. anthracis in hospital environments
• Capsule is the basis of conjugate vaccines (H. influenzae b, S. pneumoniae, N. meningitidis)
• Endotoxin (LPS) from gram-negative bacteria triggers septic shock via TLR-4

---

Summary: Bacterial structure (cell wall, membrane, appendages), classification (morphology, staining, metabolic, genetic), and growth (binary fission, four-phase growth curve) are fundamental to understanding microbial pathogenesis, diagnosis, and antibiotic treatment.