Control of Microbial Growth - Chapter 13

Unit 3: Control of Microorganisms - Study Notes

Section 1: Chapter 13 – Control of Microbial Growth

• Microbial Resistance to Control:

  • Most resistant: Prions; Bacterial endospores
  • Moderate resistance: Bacteria (e.g., Mycobacteria); Zygospores; Naked viruses
  • Least resistant: Fungal spores; Hyphae (fungal filaments); Yeasts

• Key Terms:

  • Fomite: An object that transmits disease.
  • Sterilization: Complete removal of all living microbes.
  • Sanitization: Reduction of microbes to safe levels.
  • Disinfection: Killing pathogens on nonliving surfaces.
  • Asepsis: Killing pathogens on living tissue.
  • Aseptic technique: Preventing contamination of sterile surfaces.
  • Degerming: Reducing microbes on living tissue using mild chemicals and scrubbing.
  • -cide: Kills organisms.
  • -static: Inhibits growth.
  • Microbial death: Permanent loss of reproductive capability.
  • Microbial death curve: Graph illustrating the progress of microbial control.
  • Decimal Reduction Time (D-value): Time to kill 90% of a microbial population.
  • Thermal death time: Minimum time to kill all microbes at a specific temperature.
  • Incineration: Destruction by direct flame.

• Physical Methods of Microbial Control:

  • Heat: Oxidizes cellular components.
    • Boiling: 100°C for lab preparation and personal use.
    • Dry heat oven: 170°C for 2 hours to sterilize equipment.
    • Incineration: Burns microorganisms with direct flame.
    • Autoclave: 121°C for 15 minutes, sterilizes media and equipment.
    • Pasteurization: Prevents spoilage in food at various temperatures.
  • Cold: Reduces microbial metabolic rate.
    • Freezing: Long-term storage at -2°C.
    • Refrigeration: Short-term storage at 0-7°C.
  • Desiccation (Dehydration): Removes moisture, used in food preservation.
  • Lyophilization: Rapid freezing under vacuum, used in preservation.
  • Radiation:
    • Ionizing: Gamma rays, sterilizes medical items, food, and spices.
    • Non-ionizing: UV rays, used to sterilize lab materials and water.
  • Filtration: Removes microorganisms from liquids or gases.
    • HEPA filters: Trap particles, organisms, and spores from air.
    • Membrane filters: Used in vaccine production.

• Factors Affecting Microbial Growth: Temperature, pH, moisture, nutrient availability, oxygen, and other environmental factors.

• Monitoring Autoclave Effectiveness: Autoclave tape changes color during a successful cycle.

• Filter Types:

  • Membrane filters remove microorganisms from solutions.
  • HEPA filters trap airborne particles, organisms, and spores.

Section 2: Chapter 14 – Antimicrobial Drugs

• Key Scientists:

  • Paul Ehrlich: Developed the first synthetic antimicrobial (arsphenamine).
  • Gerhard Domagk: Discovered prontosil (synthetic antimicrobial dye).
  • Alexander Fleming: Discovered penicillin from Penicillium mold.
  • Florey and Chain: Demonstrated penicillin's effectiveness against diseases.

• Types of Antimicrobial Drugs:

  • Synthetic drugs: Lab-made (e.g., prontosil).
  • Antibiotics: Naturally occurring agents that kill/inhibit organisms (e.g., penicillin).
  • Semi-synthetic: Chemically modified for enhanced activity.

• Broad/Narrow Spectrum Antibiotics:

  • Narrow spectrum: Targets one microbial group (e.g., penicillin).
  • Broad spectrum: Affects multiple groups (e.g., tetracycline). This can lead to superinfections and resistance.

• Drug Administration: Dosage and route (oral, topical, parenteral).

• Chemotherapeutic Index: Ratio of toxic dose to therapeutic dose.

• Antibiotic Targets and Classes:

  • Cell wall: β-lactams (penicillin), Glycopeptides, Bacitracin.
  • Protein synthesis: Polymyxins, Lipopeptides.
  • Ribosomes: 30S and 50S subunits (aminoglycosides, tetracyclines).
  • Metabolic pathways: Folate synthesis (sulfonamides, trimethoprim).
  • DNA synthesis: Fluoroquinolones.
  • RNA synthesis: Rifamycins.

• Antibiotic Resistance Mechanisms: Drug modification, uptake prevention, and target modification.

• Minimum Inhibitory Concentration (MIC): Lowest drug concentration inhibiting visible microbial growth.

• Susceptibility Assays:

  • Disk diffusion
  • Tube dilution
  • Microdilution tray
  • E-test

• Chemical Antimicrobial Agents:

  • Phenolics: Denature proteins, disrupt membranes (e.g., Lysol).
  • Metals: Bind to proteins, inhibit enzymes (e.g., mouthwash).
  • Halogens: Oxidize macromolecules (e.g., bleach).
  • Alcohols: Denature proteins, disrupt membranes (e.g., hand sanitizers).
  • Peroxides: Oxidize cellular components (e.g., hydrogen peroxide, benzoyl peroxide).
  • Aldehydes: Inactivate viruses, destroy bacterial cells (e.g., formaldehyde).
  • Detergents: Surfactants that disrupt membranes.
  • Gases: Sterilize materials (e.g., ethylene oxide).

• Effective Disinfectant/Antiseptic Characteristics: Kills/slows microbial growth, non-toxic to humans/animals, inexpensive, substantial shelf life.

• Evaluating Disinfectants/Antiseptics: Phenol coefficient, disk diffusion, use dilution test, in-use test