Bacterial Morphology and Structure

Bacterial Shapes & Sizes - Microscopic Menagerie

• Size Range: Typically 0.5-5.0 µm.
  • Smallest: Mycoplasma spp. (0.15-0.30 µm).
  • Largest: Thiomargarita namibiensis (up to 750 µm).
• Primary Shapes & Common Arrangements:
  • Cocci (spherical):
    • Diplococci (pairs, e.g., Neisseria gonorrhoeae, Streptococcus pneumoniae)
    • Streptococci (chains)
    • Staphylococci (grape-like clusters)
    • Tetrads (groups of 4)
    • Sarcinae (cubical packets of 8)
  • Bacilli (rod-shaped):
    • Single, diplobacilli, streptobacilli
    • Coccobacilli (short, plump rods)
    • Palisades (side-by-side, e.g., Corynebacterium diphtheriae)
  • Spiral Forms:
    • Vibrio (comma-shaped, e.g., Vibrio cholerae)
    • Spirillum (rigid, helical, external flagella)
    • Spirochete (flexible, helical, axial filament/endoflagella, e.g., Treponema pallidum)
  • Pleomorphic: Variable shape (e.g., Mycoplasma)

⭐ Mycoplasma, the smallest free-living bacteria, are unique as they lack a cell wall, making them resistant to penicillins and other cell wall synthesis inhibitors, and contributing to their pleomorphic nature.

Bacterial Cell Wall - Gram's Great Divide

Crucial for bacterial classification (Gram stain), shape, rigidity & antibiotic targets.

• Gram-Positive (+ve)
  • Thick peptidoglycan (PG) layer (up to 90%).
  • Teichoic acids (TA): Lipoteichoic (LTA) & Wall teichoic (WTA).
    • Functions: Adherence, antigenicity, autolysin regulation.
  • Stains Purple/Blue (retains Crystal Violet-Iodine).
• Gram-Negative (-ve)
  • Thin PG layer (5-10%), in periplasmic space.
  • Outer Membrane (OM) with:
    • Lipopolysaccharide (LPS) - Endotoxin:
      • Lipid A (toxic part).
      • Core polysaccharide.
      • O-antigen (somatic, variable).
    • Porins (channels).
    • Braun's lipoprotein (anchors OM to PG).
  • Stains Pink/Red (decolorized, counterstained).
• Atypical Cell Walls (Not Gram Staining)
  • Mycobacteria: Mycolic acid (Acid-fast stain).
  • Mycoplasma: No cell wall (sterols in membrane).

⭐ LPS (Endotoxin) in Gram-negative bacteria is a major virulence factor; Lipid A is responsible for its toxic effects, including fever and shock.

📌 Positive Purple. Negative piNk.

External Structures - Bacterial Bling & Armor

• Capsule (Armor): Usually polysaccharide (except Bacillus anthracis: D-glutamate polypeptide). Antiphagocytic, adherence. K antigen. Visualized: India ink, Quellung reaction.

  ⭐ 📌 SHiN: Streptococcus pneumoniae, Haemophilus influenzae type b, Neisseria meningitidis are key encapsulated bacteria; vaccines target capsular polysaccharides.

• Flagella (Motility): Protein flagellin (H antigen). Propels bacteria. Types: monotrichous, lophotrichous, amphitrichous, peritrichous.
• Pili/Fimbriae (Adherence): Glycoprotein.
  • Fimbriae (common pili): Adherence to host cells.
  • Sex Pili (F pili): Genetic transfer (conjugation).
• Glycocalyx (Slime Layer): Loose polysaccharides. Biofilm formation, adherence (e.g., Staphylococcus epidermidis on catheters).

Internal Structures & Spores - Cytoplasmic Secrets & Survival Pods

• Cytoplasm: Matrix for metabolism.
  • Ribosomes: 70S (30S+50S); protein synthesis. Targets: aminoglycosides, macrolides.
  • Nucleoid: Circular dsDNA; no nuclear membrane.
  • Plasmids: Extrachromosomal DNA; antibiotic resistance, virulence factors.
  • Inclusion Bodies: Nutrient reserves.
    • Volutin granules: Polyphosphate (C. diphtheriae).
    • Glycogen, PHB.
• Endospores: Dormant, highly resistant survival structures (not for reproduction).
  • Genera: Bacillus, Clostridium (Gram-positive).
  • Resistant to: Heat, desiccation, radiation, chemicals.
  • Layers & Components:
    • Core: DNA, ribosomes, low water.
    • Cortex: Peptidoglycan.
    • Spore Coat: Keratin-like protein.
    • Calcium dipicolinate ($Ca^{2+}$-DPA): Heat resistance.

⭐ Medically important spores (e.g., Bacillus anthracis, Clostridium tetani) are highly resistant due to calcium dipicolinate and dehydrated core.

High‑Yield Points - ⚡ Biggest Takeaways

• Peptidoglycan is unique to bacterial cell walls; a key antibiotic target.
• Gram-positive bacteria: Thick peptidoglycan, teichoic acids. Stain purple.
• Gram-negative bacteria: Thin peptidoglycan, outer membrane with LPS (endotoxin). Stain pink.
• Capsules (e.g., S. pneumoniae) are antiphagocytic virulence factors; visualized by India ink.
• Spores (Bacillus, Clostridium) are resistant dormant forms with dipicolinic acid.
• Pili/Fimbriae mediate adherence; sex pili for conjugation.
• Flagella provide motility; arrangement is characteristic for identification.