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Control of Microbial Growth

Control of Microbial Growth

Introduction

  • Control of microorganisms has been practiced for centuries.

    • Importance of microbial control in:

    • Food preservation

    • Controlling disease

    • Drinking water preservation

    • Daily actions taken to manage microbial presence discussed.

Principles of Control and Terminology

Sterilization

  • Definition: Free of all microorganisms including endospores, but not prions.

  • Achievable through:

    • Filtration

    • Heat

    • Chemicals

    • Irradiation

Disinfection

  • Definition: Eliminates most pathogens, some viable microbes may remain.

  • Disinfectants: Chemical agents used on inanimate objects/surfaces.

  • Antiseptics: Chemical agents used on living tissues.

Pasteurization

  • Definition: Brief heat treatment to reduce organisms causing food spoilage without altering product characteristics.

Decontamination

  • Definition: Reduction of pathogens to a safe level.

    • Techniques include washing, heat, or chemical treatment.

Degerming

  • Definition: Decreased number of microbes in an area, especially on skin.

Sanitized

  • Definition: Substantial reduction of microbial population to meet health standards that minimize disease spread.

    • No specific level of control defined.

Preservation

  • Definition: Delays spoilage of perishable items by adding growth-inhibiting ingredients or selecting appropriate storage conditions.

Terminologies in Control

Active Terms

  • -cide: to kill

    • Bactericide: Kills bacteria (not endospores).

    • Fungicide: Kills fungal spores, hyphae, and yeasts.

    • Virucide: Inactivates viruses.

    • Sporicide: Destroys bacterial endospores.

    • Germicide / Microbicide: Kills microorganisms.

  • Stasis / -static: To stand still

    • Bacteristatic: Prevents bacterial growth.

    • Fungistatic: Inhibits fungal growth.

    • Microbistatic: Controls microorganisms in the body.

Microbial Death

  • Begins when cellular structures fail and the cell undergoes irreversible damage.

  • A cell is in a state of microbial death when it cannot reproduce under ideal conditions.

  • Death process: Begins at a threshold of microbicidal agent and follows a logarithmic pattern (not instantaneous).

    • More time is required to destroy large populations compared to smaller ones.

    • Removing some microorganisms first can reduce sterilization/disinfection time.

Approaches to Control

Control Mechanisms

  • Can be physical, chemical, or a combination of both.

Physical Methods

  • Heat

  • Irradiation

  • Filtration

  • Mechanical removal

Chemical Methods

  • Utilize various antimicrobial chemicals based on circumstances and the required degree of control.

Variability in Microbial Control Methods

  • Factors influencing choice of microbial control methods include:

    • Daily life settings

    • Hospitals

    • Microbiology laboratories

    • Food production facilities

    • Water treatment

Selection of Antimicrobial Procedures

  • Selection is complicated; no ideal method exists.

  • Considerations include:

    • Type of microbe

    • Extent of contamination (number of organisms)

    • Environmental factors

    • Risk of infection

    • Composition of infected item

Resistance Considerations

  • Most critical factor: Is the organism resistant or susceptible?

  • Examples of resistant microbes include:

    • Bacterial endospores: Resistant to heat, drying, and chemicals.

    • Protozoan cysts/oocysts: Cause diarrheal diseases.

    • Mycobacterium species: Acid-fast walls confer resistance.

    • Pseudomonas species: Grow in disinfectants.

    • Naked viruses: More resistant than enveloped viruses.

Physical Methods to Destroy Microorganisms

Table 5.1 Overview of Physical Methods

  • Moist Heat

    • Boiling: Destroy most microorganisms; not effective against endospores.

    • Pasteurization: Reduces heat-sensitive microbes; protocol: 72°C for 15 seconds.

    • Pressurized Steam (Autoclaving): Effective against endospores; standard: 121°C at 15 psi for 15 min.

  • Dry Heat

    • Incineration: Destroys by oxidizing to ashes; used for medical waste.

    • Dry Heat Ovens: 200°C for 1.5 hours; less effective than moist heat.

  • Filtration: Removes microbes; various pore sizes (0.2 µm commonly used).

  • Radiation:

    • Ionizing Radiation: Destroys DNA; includes gamma rays, X-rays.

    • Non-ionizing Radiation: Excites atoms, generates dimers in DNA (e.g., UV light).

Ionizing Radiation

  • Directly destroys DNA and damages membranes; induces reactive oxygen species.

  • Commonly used to sterilize heat-sensitive materials.

  • Has consumer apprehension regarding irradiated products.

Non-ionizing Radiation

  • Effective for air, water, and surface disinfection; poor penetrating power; potential skin/eye damage.

Chemical Control

  • Germicidal Chemicals: React with vital cell components (proteins, DNA, membranes).

  • Grouped into:

    • Sterilants: Destroy all microorganisms.

    • High-Level Disinfectants: Destroy viruses and vegetative cells.

    • Intermediate-Level Disinfectants: Kill vegetative cells, fungi, and most viruses.

    • Low-Level Disinfectants: Remove fungi, vegetative bacteria, some viruses; not effective against all pathogens.

Selecting the Right Chemical

Factors to consider when selecting a microbicidal chemical:

  • Toxicity vs. benefits

  • Activity in the presence of organic materials

  • Compatibility with the item being treated

  • Residue potential

  • Cost and availability

  • Storage and stability

  • Environmental risks

Common Chemicals Used

Table 5.2 Overview of Antiseptic and Disinfectant Chemicals

  • Alcohols: Denature proteins; rapid evaporation limits use.

  • Aldehydes (e.g., glutaraldehyde): Effective at sterilization; irritate respiratory tract.

  • Biguanides (e.g., chlorhexidine): Low toxicity, good adherence to skin.

  • Ethylene Oxide Gas: Very effective, but toxic and explosive.

  • Halogens (e.g., chlorine, iodine): Broad-spectrum activity; can react with organic materials.

  • Ozone and Peroxygens: Readily biodegradable; effective on organic materials.

  • Phenolic Compounds (e.g., triclosan): Wide range of activity, but can be toxic.

  • Quaternary Ammonium Compounds: Non-toxic; susceptible to inactivation by soaps.

Preservation Techniques

Physical Methods

  • Low-temperature Storage: Inhibits microbial growth.

  • Freezing: Prevents enzyme activity.

  • Reducing Available Water: Involves salt/sugar addition or drying (desiccation).

  • Lyophilization: Used in food preservation processes.

Chemical Additives

  • Formaldehyde, Phenols: Used as preservatives.

  • Weak Organic Acids: (benzoic, ascorbic, propionic acids); prevent microbial growth in foods.

  • Nitrates and Nitrites: Used in meats; can be carcinogenic.

Reducing Water Availability

  • Achieved through adding salt/sugar, which causes plasmolysis in cells.

  • Some bacteria can survive in high salt (e.g., Staphylococcus aureus).

Common Antimicrobial Products

  • Overview of active ingredients and classifications across various commercial products detailed in a table format.

Practical Concerns in Microbial Control

  • Include determining whether sterilization/disinfection is necessary.

  • Consider if the item will be reused or discarded.

  • Assess the method's suitability for the intended application.

  • Ensure the agent can penetrate surfaces adequately.

  • Evaluate cost-effectiveness and safety.

Conclusion

  • Understanding the principles of microbial control is essential for effective applications in various fields including healthcare, food safety, and environmental protection.