Bacterial Cell Wall & Anatomy
โญ Foundational Microbiology Distinction
8 Parameters
Gram-Positive vs Gram-Negative Bacteria (Cell Wall Architecture)
Definitive architectural comparison of peptidoglycan thickness, outer membrane presence, lipopolysaccharides, teichoic acids, and antibiotic susceptibility.
| Structural Characteristic | Gram-Positive Bacteria | Gram-Negative Bacteria |
|---|---|---|
| Gram Stain Color | Purple / Violet (crystal violet retained) | Pink / Red (counterstained with Safranin) |
| Peptidoglycan (Murein) Layer | Thick multilayer (20 โ 80 nm, 90% of dry cell wall) | Thin single layer (2 โ 7 nm, 10% of dry cell wall) |
| Outer Membrane (OM) | ABSENT | PRESENT (asymmetric lipid bilayer with porin channels) |
| Lipopolysaccharide (LPS / Endotoxin) | Absent | Present in abundance on outer leaflet of OM (Lipid A) |
| Teichoic & Lipoteichoic Acids | Present (provide negative charge & antigenic identity) | Completely ABSENT |
| Periplasmic Space | Narrow or absent | Wide prominent periplasm containing hydrolytic enzymes |
| Susceptibility to Lysozyme & Penicillin | High (cleaves ฮฒ-1,4 glycosidic bond in exposed murein) | Low (outer membrane prevents entry; requires porin passage) |
| Flagellar Basal Body Rings | 2 Rings (M and S in cell membrane) | 4 Rings (L in LPS, P in peptidoglycan, M and S in membrane) |
Cell Wall Anatomy Comparison
Gram-Positive
Peptidoglycan: Thick (20-80 nm)
Outer Membrane: Absent
Teichoic Acids: Present
Basal Rings: 2 (M, S)
Gram-Negative
Peptidoglycan: Thin (2-7 nm)
Outer Membrane: Present
Teichoic Acids: Absent
Basal Rings: 4 (L, P, M, S)
Bacterial Toxins & Pathogenesis
โญ Medical Microbiology & Pharmacology
6 Parameters
Bacterial Toxins: Exotoxins vs Endotoxins
Comparative matrix of bacterial toxin chemical nature, heat stability, lethal dose, toxoid potential, and immune response.
| Property | Exotoxins | Endotoxins |
|---|---|---|
| Bacterial Source | Synthesized and secreted by both Gram+ and Gram- bacteria | Integral component of outer membrane of Gram- bacteria ONLY |
| Chemical Nature | Polypeptides / Proteins (enzymatic, AB subunit structure) | Lipopolysaccharide (Lipid A component is toxic moiety) |
| Thermal Stability | Heat-labile (destroyed at 60โ80ยฐC; Staph enterotoxin is exception) | Heat-stable (can withstand 100ยฐC autoclaving for 1 hour) |
| Potency & Lethal Dose (LD50) | Extremely high potency (tiny LD50 in micro/nanograms) | Low potency (requires large amounts in milligrams to cause shock) |
| Conversion to Toxoid Vaccine | Readily converted by formaldehyde into immunogenic toxoids (e.g., Tetanus, Diphtheria) | Cannot be toxoided (lipid nature destroys immunogenicity) |
| Clinical Manifestations | Highly specific clinical signs (spastic paralysis, neurotoxicity, flaccid paralysis) | Non-specific systemic symptoms: High fever, septic shock, DIC via IL-1, TNF-ฮฑ |
Toxin Class Properties
Exotoxins
Composition: Secreted Protein
Heat Stability: Heat-Labile
Toxoidable: Yes (Tetanus)
Potency: Very High (ฮผg)
Endotoxins
Composition: Lipid A (LPS)
Heat Stability: Heat-Stable
Toxoidable: No
Potency: Low (mg)
Bacterial Genetics & Gene Transfer
โญ Recombination Mechanisms
5 Parameters
Horizontal Gene Transfer: Transformation vs Transduction vs Conjugation
Mechanistic comparison of genetic transfer modes in bacteria, requirements for cell-to-cell contact, vector involvement, and DNase sensitivity.
| Parameter | Transformation (Griffith 1928) | Transduction (Zinder & Lederberg 1952) | Conjugation (Lederberg & Tatum 1946) |
|---|---|---|---|
| Gene Transfer Vector | None (naked extracellular DNA taken up directly) | Bacteriophage (lytic or temperate virus) | F-pilus / sex pilus encoded by conjugative plasmid |
| Cell-to-Cell Physical Contact | Not required | Not required | Strictly REQUIRED (mating junction via type IV secretion) |
| Sensitivity to Extracellular DNase | SENSITIVE (DNase degrades free DNA, abolishing transfer) | Resistant (DNA is protected inside phage capsid) | Resistant (DNA passes directly through conjugative bridge) |
| Cellular Competence Requirement | Requires physiological competence (Com machinery) or artificial CaCl2/heat shock | No competence needed; depends on phage surface receptor | Requires F+ / Hfr donor cell and F- recipient cell |
| Size of Transferred DNA | Small fragments (~10 โ 20 kb) | Limited by bacteriophage capsid packaging head (~40 โ 100 kb) | Can transfer entire chromosome (in Hfr strains over 100 min) |
Transfer Mechanism Key Tests
Key Point
Mode: Transformation
Dnase Sensitive: YES
Cell Contact: No
Mediator: Naked DNA
Key Point
Mode: Transduction
Dnase Sensitive: No
Cell Contact: No
Mediator: Bacteriophage
Key Point
Mode: Conjugation
Dnase Sensitive: No
Cell Contact: YES
Mediator: F-Pilus
