Microbial Control and Sterilization – Key Concepts (Flashcards)

Historical & Pre-Technological Approaches to Microbial Control

• Salting food

  • Draws out bacterial moisture (osmotic pressure) & raises pH ➔ inhospitable to microbes

  • Pre-refrigeration, widely used in diverse climates

• Smoking & fermenting

  • Dehydrates, limits oxygen, lowers pH, introduces fermentative microbes that out-compete pathogens

• Drying / freeze-drying (e.g., astronaut ice-cream, mountain-house camping meals)

  • Moisture removed ➔ shelf-life up to (30\ \text{years})

  • Re-hydration required; preserves nutrients

• UV sunlight exposure

  • Non-ionizing radiation induces thymine dimers in DNA ➔ lethal mutations

• Burning/incineration

  • Historically used for contaminated clothes, mass graves, war casualties

• Metallic storage (copper/silver vessels)

  • Oligodynamic effect: metal ions disrupt microbial membranes & enzymes; resurged during COVID (copper masks, bedding)

Essential Terminology

• Sterilization – destruction of all microbial life (e.g., surgical instruments)

• Disinfection – destruction/reduction of most microbes on inanimate objects

• Antisepsis – disinfection of living tissue (skin, mucosa)

• Decontamination / Sanitization – broad removal of most microbes on either animate or inanimate surfaces

• Asepsis – practice preventing microbe entry into sterile tissues

• Sepsis – microbial growth in blood/tissues ➔ systemic infection

• -cidal suffix (bactericidal, fungicidal, etc.) – outright kill

• -static suffix (bacteriostatic, etc.) – inhibit replication/growth

Categories of Control Methods

• Physical Agents

  • Heat (dry vs. moist)

  • Radiation (ionizing vs. non-ionizing)

• Chemical Agents

  • Gaseous (e.g., fumigation canisters reach hidden crevices)

  • Liquid (soaps, alcohols, antiseptic solutions)

• Mechanical/Removal

  • Filtration (air – HEPA; liquid – membrane, LifeStraw, RO systems)

Physical Agents: Heat

• Dry heat – ovens, incinerators (crematory near campus anecdote)

• Moist heat – boiling, pressurized steam (autoclave), pasteurization

• Autoclave parameters: \sim121^{\circ}\text{C}, 15 psi, >30 min; vault-style door to contain pressure; indicator tape stripes darken after run

• Thermal Death Time (TDT) – shortest time to kill at a given T

• Thermal Death Point (TDP) – lowest T that kills in 10 min

  • Importance: avoid resource waste & preserve product quality (analogous to correct antibiotic dose)

Pasteurization Variants

• Inactivates 97-99\% of vegetative bacteria/fungi & all viruses; does not destroy endospores or ecosystem microflora

• Ultra-pasteurized/aseptic milk ≠ “cooked” flavor; achieves safety without curdling

Sous-Vide Parallel

• Food vacuum-sealed, held \sim65{-}68^{\circ}\text{C} for >2 h ➔ uniform doneness & microbial kill (reference to instructor’s steak example)

Physical Agents: Radiation

• Non-ionizing (UV-C \approx254\ \text{nm})

  • Forms thymine dimers ➔ replication arrest

• Ionizing (X-rays, \gamma-rays)

  • Eject electrons, create radicals; used in medical disposables & food irradiation; cosmic source reminder

Chemical Agents

• Alcohols/Detergents

  • Disrupt membranes; require \geq60\% ethanol/isopropanol

• Gas (ethylene oxide, chlorine dioxide)

  • Rapid diffusion, penetrates nooks (e.g., ceiling corners)

• Liquid antiseptics (chlorhexidine, povidone-iodine)

• Metallic ions (copper, silver) impregnated surfaces/textiles

Mechanical Removal: Filtration

• Air – HEPA (Operating Rooms, cleanrooms)

• Water

  • Brita: mainly taste/odor; pore size too large for microbes

  • Reverse Osmosis: near-sterile output; wastes water

  • LifeStraw: 0.2\ \mu\text{m} membrane; blocks bacteria/protozoa; viruses rare in water, but turbidity still filtered

Microbial Resistance Spectrum (Least → Most)

1. Enveloped viruses

2. Gram-positive bacteria

3. Non-enveloped viruses

4. Fungi & fungal spores (non-endo)

5. Gram-negative bacteria

6. Protozoa (trophozoites)

7. Protozoan cysts

8. Pseudomonas & Staphylococcus (note Staph is Gram + but still hardy)

9. Mycobacteria (waxy cell wall)

10. Endospores

11. Prions (proteinaceous, no nucleic acid, extremely resistant)

• Implication: any process that kills endospores sterily eliminates all lower tiers

Factors Influencing Death Rate

• \text{N}_{0} (initial load): lower count ➔ faster control

• Microbe nature/physiology (biofilms, spore-formers, lipid envelope)

• Temperature & pH of environment (tropics favor growth)

• Concentration/dose of agent

• Exposure duration

• Interfering matter (organic soils, pus, feces)

Modes of Action of Antimicrobial Agents

• Cell wall disruption (β-lactam antibiotics, lysozyme)

• Cell membrane surfactant insertion (detergents, alcohols)

• DNA/RNA synthesis inhibition (UV, quinolones)

• Protein synthesis blockage (aminoglycosides) or enzyme denaturation (heat, heavy metals)

• Surfactants (amphipathic molecules) embed hydrophobic tail into lipid bilayer while hydrophilic head faces water, "Jenga"-like destabilization

Clinical Asepsis & Hospital Insights

• Proper gloving, sterile field prep, instrument handling critical

• HCA hospital network uses AI algorithm predicting sepsis \approx18 h earlier → 78\% mortality reduction; choose facility based on specialty (HCA for infection, Spring Valley for neuro, Summerlin/UMC/Sunrise for pediatric)

Practical Examples & Anecdotes

• Freeze-dried \approx\$2000 survival crates mistakenly ordered by dementia patient

• Needles in public spaces myth: HIV survival on dry needles very low (probability {<}0.01\%); enveloped viruses perish within minutes of air exposure

• Brita vs. LifeStraw vs. bottled \$8 airport water; cost-effective travel hydration strategy

• Crematory smoke observed near campus; environmental & ethical considerations

Ethical / Environmental Considerations

• Open-air cremation smoke vs. burial leachate: both ultimately organic decomposition; cultural sensitivities

• Overuse of antibiotics (e.g., amoxicillin in SE Asia) breeds resistance; underscores importance of correct dose & spectrum knowledge

• Resource stewardship: applying minimum effective heat/time (TDT, TDP) parallels judicious drug dosing