M

Micro 8/13/25

Pasteurization: Batch vs Flash

  • Two primary pasteurization approaches:

    • Batch method: longer exposure, higher time at a given temperature.

    • Flash method: very quick exposure to heat.

  • Both are used to reduce microbial load; flash is like sous vide for meats—lower temperature for a longer time or quicker heat with shorter exposure.

  • Key concept: exposure time is relatively more important than the temperature gradient in pasteurization.

Autoclave and Heat-Related Sterilization

  • Autoclave uses both pressure and heat simultaneously to kill microorganisms.

  • Pressure helps kill bacteria and viruses by making conditions less favorable for survival.

  • Limitations: not every material can go in an autoclave.

    • Can autoclave: glassware, cloth, metal, liquids.

    • Cannot autoclave: some plastics and materials that melt.

Bunsen Burners and Lab Heating

  • Bunsen burners are powerful but dangerous; not routinely used in some modern labs.

  • Fire and fumes: flame characteristics include a blue inner cone and an orange-red outer flame.

  • Purpose: sterilization by flame can burn contaminants to ashes.

  • Temperature control is difficult with a Bunsen burner because adjusting temperature relies on airflow, not precise temperature control.

Hot Air Oven

  • Electric oven used for sterilization by dry heat.

  • Typical operation: 100°C to 180°C for several hours.

  • Effective at destroying endospores due to prolonged exposure to heat.

Cold Treatments and Preservation

  • Cold can be used for preservation; freezing is a common preservation method.

  • Liquid nitrogen is used for ultra-cold storage.

  • Flash freezing (e.g., on ships in fisheries) rapidly freezes products to kill parasites while preserving quality.

  • Fish processing often involves flash freezing to kill parasites; parasites may still be present but dead.

  • Frozen storage in ships requires large liquid nitrogen storage and ongoing maintenance to remain viable.

  • Sashimi and frozen seafood: commercial sashimi is typically flash-frozen to inactivate parasites; if caught and sliced fresh, it is different from commercially frozen products.

  • Very low temperatures can preserve cultures, but can also destroy many organisms; accuracy of temperature is crucial.

Desiccation (Desiccation) and Drying

  • Desiccation means dehydration at room temperature.

  • Method: place sample in a container with desiccation powders (e.g., under a glass) that absorb moisture.

  • Similar to using silica gel packets in snacks to keep contents dry.

  • Important caveat: the sample must be dry before desiccation begins; moisture can hinder the process.

  • Desiccation reduces water activity, helping preserve and potentially reduce pathogens.

  • Contrast with freeze-drying: different preservation method (not described in depth here).

Radiation and Irradiation

  • Types of radiation used for microbial control:

    • Ionizing radiation: X-rays, gamma rays; causes mutations in DNA and damages repair mechanisms; generally effective at killing.

    • Nonionizing radiation: ultraviolet (UV) light; causes abnormal bonds in DNA (e.g., thymine dimers) leading to mutations.

  • Irradiation: process of bombarding material with radiation to achieve microbial kill.

  • Key difference: ionizing radiation can be more lethal to microbes; nonionizing radiation often causes mutations and cancerous changes if not repaired.

  • UV and thymine dimers: UV can cause formation of thymine dimers (TT) which interfere with DNA replication; unrepaired dimers can lead to mutations and potentially cancerous changes.

  • X-rays: general note that exposure to high levels is harmful; e.g., ~150 X-rays/year is a threshold mentioned for noticeable effects, though typical exposures are much lower.

Filtration

  • Filtration can be used for air or liquid; effectiveness depends on membrane pore size.

  • Filtration removes impurities and microbes based on what can pass through the filter.

Meat Curing, Salting, and Osmotic Preservation

  • Curing meats and high salt content are traditional preservation methods.

  • Very high salt content can inhibit microbial growth by osmotic pressure, though the transcript notes bacteria love sugar as an energy source; balance and context matter.

Chemical Disinfectants and Antiseptics

  • Chemical disinfection is one of the most commonly used methods for microbial control.

  • Subcategories include water-soluble and nonwater soluble chemicals; penetration into non-living objects is possible; some are used for critical items (e.g., catheters) that cannot be autoclaved.

  • These chemicals are typically used one-time or for limited reuse depending on the material and exposure.

  • Important notes about clinical hygiene and equipment handling include proper disinfection of devices before reuse to prevent nosocomial (hospital-acquired) infections.

  • Emphasis on proper hygiene and sterile technique in clinical settings to prevent cross-contamination between patients.

Halogens

  • Halogens include fluorine, chlorine, bromine, and iodine; they are located just before noble gases in the periodic table.

  • Iodine and chlorine are widely used; chlorine in pools is common; strong odor can indicate contamination (e.g., urine) in pools.

  • Mechanism: halogens can destroy microbial spores and other microbes; chlorine-based disinfectants release hypochlorous acid (HOCl).

  • Example modes of action: HOCl denatures enzymes and disrupts metabolic reactions; hypochlorites are basic and effective.

  • Fluoramines and chlorine-based tablets are used for water treatment to remove impurities; many waters are already chlorinated; fluoride is added for dental health in some water supplies.

  • Note: some communities are re-evaluating fluoride usage; scientific details are context-dependent.

Phenols and Phenolic Compounds

  • Benzenoid phenols (phenolics) are broad-spectrum antimicrobials; strong cellular poisons at high concentrations.

  • Mechanisms: disrupt cell walls and membranes; inactivate enzymes at lower concentrations.

  • Phenols can be used as antiseptics (e.g., throat sprays), but high concentrations are toxic; consumer products may be restricted due to toxicity concerns.

  • Commonly encountered product example: phenol-based throat sprays; do not drink.

Chlorhexidine

  • Chlorhexidine is another major antiseptic; common product: Peridex (mouthwash).

  • Uses: hand scrubbing, surgical prepping, and antiseptic application; broad action against bacteria, viruses, and fungi.

  • Advantages: low toxicity, rapid onset, and broad spectrum.

  • Often used in surgical site prepping (e.g., abdomen) and other disinfection contexts.

Iodine and Betadine (Povidone-Iodine)

  • Iodine-based products (e.g., Betadine) used for skin antisepsis; historically common for surgical scrubs.

  • Iodine is effective but can be irritant; alternative or complementary antiseptics (like chlorhexidine) are often used.

Alcohols

  • Alcohols: ethanol or isopropyl alcohol are common antiseptics.

  • Optimal concentration: around 70\%; concentrations ≥ 50\% have antimicrobial activity, but effectiveness peaks around 70\%; higher concentrations evaporate faster reducing contact time.

  • Limitations: rapid evaporation reduces exposure time; inhalation of alcohol vapors can cause health issues.

  • Note: alcohol is polar and water-soluble; helps coagulate proteins.

Oxidizing Agents

  • Hydrogen peroxide (H2O2) is widely used as an antiseptic and disinfectant.

  • Mechanism: generates reactive oxygen species (free radicals) that oxidize cellular components.

  • Common usage: 3% hydrogen peroxide for minor wounds; also used in some sterilization cabinets and professional settings.

  • Bubbles are produced when bacteria (e.g., Staphylococcus) interact with the peroxide; indicates oxidative activity.

Detergents

  • Detergents are used for cleaning and disinfection; they work on both dirt removal and microbial reduction.

  • They contain polar and nonpolar ends and may include antimicrobial components.

  • They help remove microbes from fabrics and surfaces during washing.

Silver Compounds

  • Silver nitrate and silver sulfadiazine have antimicrobial properties.

  • Broad-spectrum activity; can disrupt enzymes and proteins; effective against endospores.

  • They are relatively expensive and somewhat unstable, but historically important in burn care and other settings.

Antibiotics and Antimicrobial Therapy

  • Antimicrobials are drugs used to control infections; not perfect—no perfect drug exists.

  • Ideal antimicrobial would destroy the infective agent without harming the host (bactericidal, not bacteriostatic).

  • Side effects and patient compliance are critical: lower side effects and easier dosing improve adherence.

  • Cost considerations include insurance coverage and affordability; care models increasingly focus on value-based care (outcomes-based reimbursement).

  • Key pharmacodynamic concepts from the transcript include:

    • Narrow-spectrum vs broad-spectrum antibiotics: start with narrow spectrum when possible; broad spectrum when the pathogen is unknown.

    • Bioavailability: highest with intravenous (IV) administration (100% bioavailability). Other routes have variable absorption and distribution.

    • Delivery to the site of infection: topical, systemic (oral or IV).

    • Prophylaxis: preventive antibiotic use before surgeries to reduce intraoperative infections.

    • The goal is to achieve effective concentrations at the site of infection while minimizing toxicity and resistance development.

Drug Properties and Practical Considerations

  • Important properties of antimicrobials:

    • Solubility: water solubility improves bioavailability.

    • Potency and stability in biological environments.

    • Tissue penetration and ability to cross barriers (e.g., blood-brain barrier).

    • Resistance profiles: agents with less resistance are preferred when possible.

  • Delivery routes:

    • IV (intravenous) provides the highest bioavailability (100%).

    • Oral and topical routes have variable systemic absorption.

  • Dosing strategies:

    • The minimum effective dose should be used to minimize toxicity and resistance while achieving the needed effect.

    • Dosing decisions may rely on susceptibility testing and pharmacodynamics.

Narrow vs Broad Spectrum and Susceptibility Testing

  • Narrow-spectrum antibiotics target specific groups of microbes (e.g., gram-positive) and are preferred when the pathogen is known.

  • Broad-spectrum antibiotics affect a wide range of microbes (both gram-positive and gram-negative).

  • Initial identification often relies on clinical presentation and knowledge of common pathogens in a given site (e.g., strep throat likely caused by gram-positive Streptococcus).

  • Susceptibility testing is essential to confirm which antibiotics will be effective.

  • Two main evidence-based approaches used in labs:

    • Kirby-Bauer disk diffusion test: place antibiotic discs on a bacterial culture and observe the zone of inhibition.

    • E-test (gradient diffusion test): uses a strip with a concentration gradient to find the minimum inhibitory concentration (MIC) where growth is inhibited.

  • How to interpret:

    • Zone of inhibition: the clear area around a disc indicates effectiveness; the size depends on the drug, organism, and conditions.

    • An example: if the disc shows no growth around it at 9:00 position but growth elsewhere, that disc is effective or not depending on the exact zones; a chart is used to interpret exact MICs and zones.

  • The MIC (minimum inhibitory concentration) is the lowest concentration that visibly inhibits growth in the test tube or well and guides dosing decisions.

  • Dilution methods (broth dilution or microdilution) also determine MIC by progressively diluting the drug and observing growth.

  • Practical clinical reasoning:

    • In some infections (e.g., strep throat), a clinician may treat empirically with a gram-positive antibiotic (e.g., amoxicillin) without waiting for full culture confirmation because presentation strongly suggests a typical organism.

    • If cultures show sensitivity, clinicians can tailor therapy to the susceptible antibiotic.

Clinical Practice, Hygiene, and Sterile Technique

  • Emphasizes proper hygiene and sterile technique in clinical settings to prevent nosocomial infections.

  • Sterile field concepts:

    • One person maintains a sterile field; another handles non-sterile items.

    • Packages’ outside surfaces are contaminated; only the inside is sterile after opening.

    • Gloves and sterile gowns are used to maintain asepsis during procedures.

  • Specific examples:

    • Foley catheter handling requires sterile technique to prevent infections.

    • Reusing respiratory equipment requires disinfection to prevent cross-contamination between patients.

  • Improper hygiene and practice can lead to spread of resistant pathogens; the speaker emphasizes that proper practice must be demonstrated and checked off before working with real patients.

Additional Context and Aneidotes from the Lecture

  • A few tangential notes touched on:

    • The existence of a “frozen zoo” at the San Diego Zoo’s research facilities, housing DNA from extinct species in liquid nitrogen for potential future revival.

    • A casual aside about the prevalence of spaghetti and its rapid bacterial growth under certain conditions, illustrating the ubiquity of bacteria in food and the importance of food safety.

    • The idea that some medications and procedures are used in one-time or limited-use scenarios due to toxicity, material constraints, or risk of side effects.

Summary of Key Formulas and Numerical References

  • Pasteurization temperature exposure relationships are expressed as time vs temperature; explicit formulas not provided, but the principle is exposure time matters more than a gradient.

  • Temperature ranges mentioned:

    • Hot air oven: T \in [100^{\circ}C, 180^{\circ}C] for several hours.

  • Alcohol concentrations:

    • Optimal antiseptic concentration: 70\%

    • Minimum effective concentration: \ge 50\%

  • Radiation concepts:

    • Ionizing radiation vs nonionizing radiation (no explicit equations, but distinctions explained).

    • UV causes thymine dimers: TT dimers disrupt DNA replication.

  • Susceptibility testing:

    • Kirby-Bauer disk diffusion: zone of inhibition around antibiotic discs on culture.

    • E-test (gradient diffusion): MIC determined at the point where inhibition touches the strip.

    • MIC (minimum inhibitory concentration): the lowest antibiotic concentration that visibly inhibits growth.

  • Notable thresholds:

    • Radiation exposure threshold mentioned: about 150 X-rays per year for noticeable effects.

Connections to Foundational Principles and Real-World Relevance

  • Pasteurization, autoclaving, and other sterilization methods are foundational to microbiology, food safety, and healthcare infection control.

  • Radiation, desiccation, and freezing reflect physical methods of controlling microbes and preserving biological materials.

  • Chemical disinfectants and antiseptics (halogens, phenolics, chlorhexidine, alcohols, oxidizers) are core to hospital infection control, wound care, and consumer hygiene.

  • Antibiotics and antimicrobial therapy sit at the intersection of microbiology, pharmacology, and clinical medicine; success depends on choosing appropriate agent, dose, delivery method, patient factors, and resistance dynamics.

  • Laboratory susceptibility testing (Kirby-Bauer, E-tests, dilution methods) bridges microbiology with clinical decision-making, guiding targeted therapy and stewardship.

  • Proper hygienic practices and sterile technique are essential to prevent nosocomial infections and ensure patient safety in clinical settings.