🧬 Bacterial cell envelope
Gram-positive and Gram-negative bacteria share a cytoplasmic membrane and a peptidoglycan layer, but differ fundamentally in outer structure. The envelope is the primary interface with the host and the target of many antibiotics.
Gram-positive
- Thick peptidoglycan (many layers) — retains crystal violet stain.
- Teichoic acids (wall & lipoteichoic) — adhesion and immunomodulation.
- No outer membrane; periplasm is minimal.
- Examples: Staphylococcus, Streptococcus, Bacillus.
Gram-negative
- Thin peptidoglycan (1–2 layers) — does not retain crystal violet.
- Outer membrane containing lipopolysaccharide (LPS).
- LPS = lipid A (endotoxin) + core polysaccharide + O-antigen.
- Examples: E. coli, Pseudomonas, Neisseria.
📌 Lipid A — the toxic moiety of LPS, binds TLR-4 on macrophages, triggers cytokine release (TNF-α, IL-1, IL-6). It is heat-stable and weakly immunogenic.
🧪 Peptidoglycan synthesis
Peptidoglycan is a mesh of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by short peptides. Synthesis occurs in three major stages:
Cytoplasm: NAG-NAM-pentapeptide
→
Membrane: bactoprenol carrier
→
Periplasm: transglycosylase → chain elongation
→
Transpeptidase (PBP) → cross-linking
- Transpeptidases (penicillin-binding proteins, PBPs) catalyze the final cross-linking step, removing terminal D-alanine.
- The third amino acid in the peptide (e.g., diaminopimelic acid or L-lysine) forms the interpeptide bridge.
- Cross-linking provides rigidity and resistance to osmotic lysis.
⏳ Clinical pearl: β-lactam antibiotics inhibit PBPs, blocking transpeptidation. Vancomycin binds D-Ala-D-Ala termini, preventing cross-linking.
💊 Antibiotics targeting cell wall
| Drug | Mechanism | Resistance |
|---|---|---|
| Bacitracin | Interferes with bactoprenol pyrophosphate recycling | — |
| β-lactams (penicillins, cephalosporins) | Bind PBPs → inhibit transpeptidase | β-lactamases, altered PBP, porin loss |
| Vancomycin | Binds D-Ala-D-Ala → blocks transpeptidation | VanA/VanB: alter terminal D-Ala-D-Lac |
💊 High-yield: β-lactams are most effective during log phase (active growth). Gram-negative bacteria have additional resistance due to outer membrane porin restriction.
📈 Bacterial growth curve & culture
Phases
- Lag — adaptation, enzyme synthesis; no increase in viable count.
- Log (exponential) — optimal growth, generation time determined; antibiotics most effective.
- Stationary — nutrient depletion, waste accumulation; cell death = new cells.
- Death — irreversible decline.
Culture media (selected)
- Chocolate agar — Neisseria, Haemophilus
- Thayer-Martin — selective for pathogenic Neisseria
- MacConkey / EMB — enteric differential/selective
- Lowenstein-Jensen — Mycobacterium
- Buffered charcoal yeast extract (BCYE) — Legionella
🧫 Obligate intracellular (viruses, Rickettsia, Chlamydia) require cell culture or embryonated eggs. Treponema pallidum cannot be cultured in vitro.
🦠 Pathogenicity & virulence
Colonization & adherence
- Pili/fimbriae — primary adhesins in Gram-negative bacteria.
- Teichoic acids — mediate attachment in Gram-positives.
- Biofilms — Staphylococcus epidermidis, Pseudomonas on inert surfaces.
Evasion of host defenses
- Capsules — anti-phagocytic (e.g., S. pneumoniae, Klebsiella).
- M protein of S. pyogenes, protein A of S. aureus.
- IgA proteases — Neisseria, Haemophilus, S. pneumoniae.
- Antigenic variation — pili and OMP in N. gonorrhoeae; phase variation in enterics.
Intracellular survival
- M. tuberculosis — inhibits phagosome-lysosome fusion.
- Listeria — escapes phagosome into cytosol.
- Type III secretion systems — inject effectors directly into host cells (e.g., Yersinia, Salmonella, E. coli).
⚠️ Immune-mediated damage: rheumatic fever (cross-reactive antibodies), granulomatous inflammation in TB/leprosy, post-streptococcal glomerulonephritis (immune complexes).
☣️ Toxins: endotoxin & exotoxins
Endotoxin (LPS)
- Lipid A — released upon bacterial lysis.
- Activates macrophages → TNF-α, IL-1, IL-6.
- Fever, hypotension, DIC, shock.
- Heat-stable, weakly immunogenic; no toxoid.
Exotoxins (proteins)
- Secreted by both Gram-positive and Gram-negative.
- Highly toxic, immunogenic → toxoid vaccines.
- Often A-B toxins (B = binding, A = enzymatic).
- Subtypes: enterotoxins, neurotoxins, cytotoxins.
Selected exotoxins
| Organism | Toxin | Mechanism | Effect |
|---|---|---|---|
| Corynebacterium diphtheriae | Diphtheria toxin | ADP-ribosylates eEF-2 | Inhibits protein synthesis |
| Pseudomonas aeruginosa | Exotoxin A | ADP-ribosylates eEF-2 | Liver damage, protein synthesis inhibition |
| Shigella dysenteriae | Shiga toxin | 60S ribosome inactivation | Enterotoxic, cytotoxic |
| Clostridium tetani | Tetanus toxin | Blocks inhibitory (GABA/glycine) release | Spastic paralysis |
| Clostridium botulinum | Botulinum toxin | Blocks ACh release | Flaccid paralysis |
| Vibrio cholerae | Cholera toxin | ADP-ribosylates Gs → ↑cAMP | Profuse watery diarrhea |
| Bacillus anthracis | Anthrax toxin (EF, LF, PA) | EF = adenylate cyclase; LF = lethal | Edema, cell death |
| Staphylococcus aureus | TSST-1 (superantigen) | TCR/MHC cross-linking | Toxic shock syndrome |
🧬 Superantigens: TSST-1 and streptococcal exotoxin A cause massive T-cell activation, cytokine storm, rash, and shock.
📊 Comparison: Gram-positive vs Gram-negative
| Feature | Gram-positive | Gram-negative |
|---|---|---|
| Peptidoglycan | Thick (many layers) | Thin (1–2 layers) |
| Outer membrane | Absent | Present (LPS) |
| Teichoic acids | Present | Absent |
| Periplasmic space | Narrow | Wide |
| Susceptibility to penicillin | Generally higher | Lower (porin barrier + β-lactamases) |
| Endotoxin | No | Yes (lipid A) |
✔️ All concepts synthesized independently — educational use only
high‑yield