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Physical & Chemical Control of Microbes

Historical Background & Traditional Practices

  • Before modern refrigeration and technology, societies used various physical and chemical tactics to slow microbial growth and prolong shelf-life.

    • Salting

    • Draws water out of bacterial cells (osmosis), creating a hypertonic environment.

    • Raises pH, further stressing microbes.

    • Smoking

    • Dries food, removes oxygen, introduces antimicrobial compounds from smoke, and can initiate fermentation.

    • Drying / Freeze-drying

    • Removes moisture entirely; products re-hydrate when water is added (e.g., 30-year-shelf-life camping meals).

    • Sunlight exposure

    • Ultraviolet (UV) light induces thymine (T) dimers in DNA, leading to lethal mutations.

    • Burning / Incineration

    • Historically used for corpses or contaminated materials to prevent epidemic spread (e.g., wartime mass graves).

    • Metal storage containers (Cu & Ag)

    • Copper and silver ions interfere with bacterial membranes & enzymes; modern marketing of copper masks, bedding, utensils during COVID-19 relies on this principle.

Core Vocabulary: Levels of Control

  • Sterilization

    • Destruction/removal of all microbial life (including endospores & viruses).

    • Example: Operating-room (OR) instruments.

  • Disinfection

    • Reduces/ destroys most microbial life on inanimate objects (tables, chairs, Lysol).

  • Antisepsis

    • Disinfection on living tissue (skin wipes, mouthwash).

  • Decontamination / Sanitization

    • Broad term: removal of most microbes from either animate or inanimate surfaces (e.g., hand-sanitizer use).

Three Broad Categories of Modern Control Methods

  1. Physical Agents

    • Incineration (dry heat)

    • Moist heat (boiling, steam under pressure, pasteurization)

    • Radiation (UV)

  2. Chemical Agents

    • Gaseous (e.g., ethylene oxide fumigation reaches hidden crevices)

    • Liquid (sprays, hand soaps, alcohol gels)

  3. Mechanical Removal

    • Filtration

      • Air: HEPA filters in ORs.

      • Liquid: Water filters (membrane size determines exclusion; Brita = taste only, LifeStraw = \approx 0.2 μm, reverse-osmosis best).

Relative Resistance of Microbes (Most → Least)

  • Prions (proteins only, highly resistant)

  • Bacterial endospores

  • Mycobacteria (waxy wall)

  • Pseudomonas & Staphylococcus (noted exception: Staph = Gram + but still hardy)

  • Protozoan cysts > trophozoites

  • Gram-negative bacteria

  • Fungal spores (non-endo)

  • Non-enveloped viruses

  • Gram-positive bacteria

  • Enveloped viruses (least resistant; quickly die on surfaces)

Endospores & Sterilization Goal

  • Endospores = benchmark for sterilization; if a process kills spores, it kills everything else beneath.

  • Disinfectants do not reliably destroy spores; only validated sterilization cycles do.

Heat Parameters & Definitions

  • Thermal Death Time (TDT): shortest time required to kill a microbe at a specified temperature.

  • Thermal Death Point (TDP): lowest temperature that kills microbes in a given time.

  • Duration often more critical than temperature (e.g., 100\,^{\circ}\text{C} for 1 h can be superior to 150\,^{\circ}\text{C} for 1 min).

Pasteurization

Method

Temp

Time

Key Notes

Flash

71.6\,^{\circ}\text{C}

15 s (x2 cycles)

Widely used for milk & juice

Batch

63\text{–}66\,^{\circ}\text{C}

30 min

Older method

  • Inactivates most viruses; destroys \approx97\text{–}99\% vegetative bacteria & fungi.

  • Does NOT kill endospores or heat-resistant microbes.

Steam Under Pressure (Autoclave)

  • Combines moist heat & high pressure.

  • Standard cycle: 121\,^{\circ}\text{C}, 15 psi, 15 min (varies by load).

  • Safety: chamber built like vault; indicator tape turns striped/black when proper conditions reached.

Sous-Vide Example (Connection)

  • Culinary version of precise moist-heat control: food vacuum-sealed & held at controlled \small165\,^{\circ}\text{F} (~74\,^{\circ}\text{C}).

Surfactants, Detergents & Alcohols

  • Surfactant = amphipathic molecule (hydrophobic tail + hydrophilic head).

    • Inserts into lipid bilayers → disrupts integrity → leakage → cell death.

  • Ordinary soap acts as surfactant; alcohol (≥ 60 %) further denatures proteins & dissolves membranes.

Static vs. Cidal Terminology

  • -cidal = kills (germicidal, bactericidal, sporicidal, virucidal, fungicidal).

  • -static = inhibits growth/reproduction (bacteriostatic antibiotics slow division so immune system clears infection).

Factors Influencing Antimicrobial Death Rate

  1. Microbial load (initial # of organisms).

  2. Population composition (type, resistance profile).

  3. Environmental conditions (temperature, pH, organic matter, moisture).

  4. Concentration & mode of action of agent (e.g., membrane disruptor vs. DNA inhibitor).

  5. Duration of exposure—insufficient time may spare resistant cells.

Mechanisms of Action (Targets)

  • Cell wall synthesis / integrity (e.g., β-lactams).

  • Cell membrane (detergents, alcohol, polymyxins).

  • Protein structure & function (heat, alcohol, heavy metals → denature enzymes).

  • Nucleic acid synthesis (radiation, quinolones).

  • Metabolic pathways (sulfonamides block folate production).

Clinical & Real-World Implications

  • Knowing hospital specializations increases patient survival (e.g., HCA’s sepsis algorithm; Spring Valley for neuro; UMC/Sunrise/Summerlin for pediatrics).

  • Rapid treatment reduces microbial load before exponential growth peaks.

  • Overuse of antibiotics (e.g., amoxicillin in SE Asia) drives resistance → drug choices differ by region.

  • Needle-stick or condom surface HIV transmission risk extremely low because enveloped viruses perish quickly in air.

Ethical & Safety Notes

  • Mass cremations historically minimized disease but raise cultural & emotional issues.

  • Biocide gas deployment reaches hidden crevices yet poses inhalation risks for humans → strict OSHA protocols.

Quick Reference Temperatures

  • UV damage: no temperature change but \lambda \approx 260\text{–}280\,\text{nm} forms thymine dimers.

  • Boiling water: 100\,^{\circ}\text{C} (kills most vegetative cells in ~10 min but not spores).

  • Autoclave: 121\,^{\circ}\text{C} @ 15 psi → sterilization.

  • Pasteurization flash: 71.6\,^{\circ}\text{C}, 15 s.

  • Sous-vide steak: \approx74\,^{\circ}\text{C} for 2 h yields safe medium-rare after surface sear.

Recap Checklist for Exams

  • Distinguish sterilization vs. disinfection vs. antisepsis vs. decontamination.

  • Recall order of microbe resistance (prion → enveloped virus).

  • Identify mechanisms (cell wall, membrane, proteins, DNA).

  • Explain TDT vs. TDP; cite pasteurization values.

  • Describe surfactant action & difference from pulmonary surfactant.

  • Know conditions for autoclaving & indicator tape purpose.

  • Differentiate ‑cidal vs. ‑static agents.

  • Apply real-world examples: copper surfaces, LifeStraw, sous-vide, hospital selection.