Microbiology Basics – Bacteria, Viruses & Fungi

Bacteria: General Characteristics

• Enormous diversity of species; each distinguished by:
  • Shape & size (morphology)
  • Nutritional needs (carbon, nitrogen, trace elements, energy source)
  • Oxygen tolerance (aerobic, anaerobic, facultative, micro-aerophilic)
  • Temperature, pH, moisture, osmotic pressure preferences
  • Activity/role (pathogenic, commensal, environmental, industrial)
• Key take-away: No single description fits all bacteria; classification relies on multiple observable and physiological traits.

Gram Staining (Differential Stain)

• Purpose: Rapid preliminary ID; guides antibiotic choice.
• Process (Crystal-violet → Iodine → Alcohol → Safranin):
  1. Cells fixed to slide.
  2. Crystal-violet stains all bacteria.
  3. Iodine mordant forms CV-I complex.
  4. Alcohol decolorises Gram-negative walls (thin peptidoglycan + outer LPS), retains Gram-positive walls (thick peptidoglycan).
  5. Safranin counterstains decolorised cells.
• Colour key:
  • Gram-positive ⇒ blue/purple (CV retained)
  • Gram-negative ⇒ red/pink (safranin)

Morphology (Shape) = “Cellular Architecture”

• Three canonical forms (remember the mnemonic “Co-Ba-Spi”):
  1. Cocci (spherical / round)
  • Memory trick: the word "cocci" contains round letters C, O.
  1. Bacilli (rod-shaped)
  • Memory trick: the letters I and l resemble rods.
  1. Spirilla (spiral / helical)
  • Memory trick: “spir” in the name signals spiral.
• Microscopic appearance:
  • Cocci often cluster (e.g., Staphylococcus) or chain (e.g., Streptococcus).
  • Bacilli may be single, pairs, chains (e.g., Bacillus, E. coli).
  • Spirilla typically appear as corkscrews or wave-forms (e.g., Treponema).
• Important: RBCs are ~ 7\,\text{µm} in diameter and can be mistaken for cocci at low magnification; note size discrepancy.

Growth Requirements & Kinetics

• Five major physical/chemical factors controlling growth:
  1. Temperature (psychrophile, mesophile, thermophile ranges)
  2. pH
  3. Oxygen presence/absence
  4. Moisture / water activity
  5. Nutrient availability (carbon, energy, trace elements)
• Doubling/Generation time:
  • Under optimal conditions many bacteria double every \approx 20\,\text{min}.
• Human-associated bacteria typically tolerate 5.5 \leq \text{pH} \leq 8.5;
  • Optimal at \text{pH} = 7.0 (near neutral).

Controlling Bacterial Growth

• Strategies
  1. Prevent multiplication (static)
  2. Kill cells outright (cidal)
• Physical methods: heat (moist/dry), autoclave, filtration, radiation, mechanical rubbing.
• Chemical methods: disinfectants, antiseptics, antibiotics (selective toxicity), pH alteration.
• Relevance: infection control, sterilization protocols, food preservation, public-health policy.

Viruses: Contrasting Features

• Size: Nanometre scale; much smaller than bacteria.
• Obligate intracellular parasites → must find a host cell to reproduce.
• Life-cycle steps:
  1. Adsorption to specific receptor on host.
  2. Penetration/entry.
  3. Uncoating (capsid removal) releasing nucleic acid.
  4. Biosynthesis: viral nucleic acid + viral enzymes hijack host machinery.
  5. Assembly.
  6. Release (lysis, budding, persistent infection, cell transformation).
• Terminology note: “Virus” derives from Latin for “poison.”
• Pathogenesis: kills or alters host cells → disease manifestations.
• Control difficulties:
  • Antibiotics ineffective (target bacterial structures absent in viruses).
  • Inside body → must rely on immune response, antivirals, or vaccines.
  • Outside body → susceptible to high heat, irradiation, strong chemicals.

Fungi: The Third Major Group

• Includes molds, mushrooms, yeasts.
• Yeasts are part of normal human flora; usually harmless until overgrowth (opportunistic).
• Medically relevant examples:
  • Candida albicans → thrush, candidiasis.
  • Dermatophytes (“ringworm,” athlete’s foot) → skin infections.
• Growth parallels bacteria (need moisture, appropriate pH/temperature) but cell wall composition (chitin) & eukaryotic status ≠ bacteria.
• Treatment: antifungals (azoles, polyenes) not antibacterials.

Comparative Snapshot

• Bacteria: free-living, prokaryotic, divide by binary fission, Gram ±, affected by antibiotics.
• Viruses: acellular, need host, no metabolism, life-cycle stages, unaffected by standard antibiotics.
• Fungi: eukaryotic, spores/yeast budding, opportunistic, susceptible to antifungal drugs.

Practical / Ethical / Real-World Connections

• Clinical decision-making: Gram stain guides empiric therapy within minutes.
• Public-health: sterilization & disinfection protocols stem from understanding growth conditions.
• Antibiotic stewardship: misuse fosters resistance; knowing bacterial vs viral etiology prevents unnecessary prescriptions.
• Infection-control ethics: obligation to minimise iatrogenic infections by adhering to physical/chemical control measures.

Memory Aids & Exam Tips

• Shape mnemonic: “Co-Ba-Spi = Round-Rod-Spiral.”
• Gram-positive ≈ "Purple-Positive" (both start with P).
• Optimal human pathogens: 37^{\circ}\text{C}, \text{pH}\,7 → expect questions on why fever, acidity affect growth.
• Generation-time math: If one cell doubles every 20\,\text{min}, after n minutes cell count N = 2^{n/20}.
• Distinguish control goals: -static (inhibit) vs -cidal (kill).