Chapter 11_FA23_V2.ppt

Chapter 11: Physical and Chemical Agents for Microbial Control

Controlling Microorganisms

• Methods for Decontamination

  • Involves physical, chemical, and mechanical techniques to destroy or reduce undesirable microbes in a given area.

  • Targets microorganisms capable of causing infection or spoilage:

    • Vegetative bacterial cells and endospores

    • Fungal hyphae and spores, yeast

    • Protozoan trophozoites and cysts

    • Worms

    • Viruses

    • Prions

Relative Resistance of Microbes

• Resistance Hierarchy:

  • Highest Resistance:

    • Prions

    • Bacterial endospores

  • Moderate Resistance:

    • Pseudomonas sp.

    • Mycobacterium tuberculosis

    • Staphylococcus aureus

    • Protozoan cysts

  • Least Resistance:

    • Most bacterial vegetative cells

    • Fungal spores and hyphae

    • Yeast

    • Enveloped viruses

    • Protozoan trophozoites

The Resistance of Endospores

• Control Agents Resistance:

  • Heat (moist):

    • Endospores: 120°C

    • Vegetative Forms: 80°C

  • Radiation (X-ray):

    • Endospores: 4000 grays

    • Vegetative Forms: 1000 grays

  • Sterilizing Gas:

    • Endospores: 1200 mg/L

    • Vegetative Forms: 700 mg/L

  • Sporicidal Liquid (2% glutaraldehyde):

    • Endospores: 3 hours

    • Vegetative Forms: 10 min

Terminology and Methods of Control

• Sterilization:

  • Destroys all viable microbes, including viruses and endospores.

• Disinfection:

  • Destroys vegetative pathogens but not endospores; applied to inanimate objects.

• Antiseptic:

  • Disinfectants applied directly to exposed body surfaces.

• Sanitization:

  • Cleansing techniques that mechanically remove microbes.

Factors Affecting Death Rate

• Influencing Factors:

  • Number of microbes present

  • Nature of microbes in the population

  • Temperature and pH of the environment

  • Concentration or dosage of agent

  • Mode of action of the agent

  • Presence of solvents, organic matter, or inhibitors

Practical Concerns in Microbial Control

• Selection Criteria:

  • Application requirement for sterilization?

  • Will the item be reused?

  • Can the item withstand heat, pressure, radiation, or chemicals?

  • Suitability of the method

  • Penetration effectiveness of the agent

  • Cost, labor efficiency, and safety of the method

Mode of Action of Antimicrobial Agents

• Cellular Targets:

  1. Cell Wall:

     • Becomes fragile; example agents include some antimicrobial drugs and detergents.

  2. Cell Membrane:

     • Phospholipid bilayer loses integrity due to surfactants.

  3. Protein and Nucleic Acid Synthesis:

     • Prevention of replication, transcription, translation; example agents include chloramphenicol and formaldehyde.

  4. Proteins:

     • Disruption and denaturation; affected by agents like alcohols, phenols, and heat.

Methods of Physical Control

• Methods include:

  1. Heat (moist and dry)

  2. Cold temperatures

  3. Desiccation

  4. Radiation

  5. Filtration

Heat Control Methods

• Moist Heat Methods:

  • Steam under pressure (e.g., autoclave, 121°C for 15-40 min).

  • Boiling at 100°C to destroy non-spore forming pathogens.

• Dry Heat:

  • Incineration or dry ovens at temperatures of 150–180°C.

Pasteurization

• Process:

  • Heat applied to eliminate potential infection agents without compromising food value.

  • Common methods:

    • 63°C for 30 min (batch method)

    • 71.6°C for 15 seconds (flash method)

  • Not classified as sterilization.

Cold Methods

• Effects of Cold:

  • Microbiostatic, slowing microbial growth.

  • Used for food and culture preservation.

Radiation Methods

• Ionizing Radiation:

  • Deep penetrating power; used for sterilizing medical supplies.

• Nonionizing Radiation:

  • UV light creates pyrimidine dimers that impede replication; directly exposed of limited penetrating power.

Filtration

• Process:

  • Physical removal of microbes by filtering liquids or gases.

• Applications:

  • Sterilizing heat-sensitive liquids and air.

Chemical Agents of Microbial Control**

• Levels of Chemical Decontamination:

  • High-Level Germicides:

    • Kill endospores; used in sterile environments.

  • Intermediate-Level:

    • Kill fungal spores, tubercle bacillus, viruses; disinfect devices for mucous membrane contact.

  • Low-Level:

    • Eliminate vegetative bacteria and some viruses; clean surfaces in contact with skin.

Factors Affecting Germicidal Activity of Chemicals

• Variables include:

  • Nature of materials treated

  • Degree of contamination

  • Exposure time

  • Strength and action of the germicide

Specific Chemical Agents

• Alcohols:

  • Ethyl and isopropyl, effective against bacteria and many endospores, with intermediate-level activity.

• Hydrogen Peroxide:

  • Produces hydroxyl radicals that damage proteins and DNA.

• Quaternary Ammonium Compounds (Quats):

  • Act as surfactants, leading to low-level decontamination.

• Soaps and Detergents:

  • Mechanically remove soil and microbes.

Concept Checks**

• Example Checks Include:

  • What method accomplishes antimicrobial effects by creating nucleotide dimers? (C. UV light)