Chapter 7: The Control of Microbial Growth – Comprehensive Study Notes

Chapter 7: The Control of Microbial Growth

The Terminology of Microbial Control

Understanding the terminology of microbial control is essential for microbiology students, as it forms the basis for discussing methods to prevent, inhibit, or destroy microorganisms in various environments.

• Sepsis: Refers to bacterial contamination, often associated with infection.

• Asepsis: The absence of significant contamination; aseptic techniques are crucial in surgery to prevent infection.

• Sterilization: The process of removing and destroying all microbial life, including endospores.

• Commercial Sterilization: Specifically targets Clostridium botulinum endospores in canned goods.

• Disinfection: Destroys harmful microorganisms on inanimate surfaces.

• Antisepsis: Destroys harmful microorganisms from living tissue.

• Degerming: Mechanical removal of microbes from a limited area (e.g., hand washing).

• Sanitization: Reduces microbial counts on eating utensils to safe levels.

• Biocide (germicide): Treatments that kill microbes.

• Bacteriostasis: Inhibits, but does not kill, microbes.

Microbial Death Rate and Factors Affecting Control

The effectiveness of microbial control methods depends on several factors, including the number of microbes, environmental conditions, exposure time, and microbial characteristics.

• Microbial Exponential Death Rate: Microbes die at a constant rate when exposed to a lethal agent.

• Key Factors:

  • Number of microbes present

  • Environment (organic matter, temperature, biofilms)

  • Time of exposure

  • Microbial characteristics (e.g., endospores, cell wall structure)

Actions of Microbial Control Agents

Microbial control agents act by damaging cellular structures and functions.

• Damage to plasma membrane: Causes leakage of cellular contents and interferes with cell growth.

• Damage to proteins (enzymes): Denaturation leads to loss of function.

• Damage to nucleic acids: Prevents replication and transcription.

Physical Methods of Microbial Control

Physical methods are widely used to control microbial growth, especially in healthcare and food industries.

Heat

• Heat denatures enzymes: Disrupts metabolic processes.

• Thermal Death Point (TDP): Lowest temperature at which all cells in a liquid culture are killed in 10 minutes.

• Thermal Death Time (TDT): Minimal time for all bacteria in a liquid culture to be killed at a particular temperature.

• Decimal Reduction Time (DRT): Minutes to kill 90% of a specific population of bacteria at a given temperature.

Moist Heat Sterilization

• Coagulates/denatures proteins: Effective for most organisms.

• Boiling and free-flowing steam: Used for disinfection.

• Autoclave: Steam under pressure (121°C at 15 psi for 15 minutes) kills all organisms (except prions) and endospores. Steam must contact the item’s surface.

• Sterilization indicators: Test strips are used to confirm sterility.

• Pasteurization: Reduces spoilage organisms and pathogens in milk and juices. High-temperature short-time (HTST): 72°C for 15 sec. Thermoduric organisms survive but are unlikely to cause disease.

• Ultra-high-temperature (UHT): 140°C for 4 seconds, followed by rapid cooling; sterilizes milk and juices for storage without refrigeration.

• Dry Heat Sterilization: Kills by oxidation (flaming, incineration, hot-air oven at 170°C for 2 hours).

Filtration

Filtration is used for heat-sensitive materials, such as culture media, vaccines, and antibiotics.

• HEPA filters: Remove microbes >0.3 μm in diameter.

• Membrane filters: Remove microbes >0.22 μm; pore sizes as small as 0.05 μm can filter out viruses and large proteins.

Other Physical Methods

• Low temperature: Bacteriostatic effect (refrigeration, deep-freezing, lyophilization).

• High pressure: Denatures proteins and alters carbohydrate structure.

• Desiccation: Absence of water prevents metabolism.

• Osmotic pressure: High concentrations of salts and sugars create a hypertonic environment, causing plasmolysis.

Radiation

• Ionizing radiation: (X-rays, gamma rays, electron beams) ionizes water to create reactive hydroxyl radicals, damaging DNA by causing lethal mutations. Used for sterilization of food, pharmaceuticals, and medical supplies.

• Nonionizing radiation: (Ultraviolet, 260 nm) damages DNA by creating thymine dimers. UVC lamps are used for surface sterilization in hospitals.

• Visible blue light (470 nm): Kills bacteria by forming singlet oxygen.

• Microwaves: Kill by heat, not especially antimicrobial.

Chemical Methods of Microbial Control

Chemical agents are used to disinfect, sanitize, and sterilize surfaces, instruments, and living tissues.

Principles of Effective Disinfection

• Concentration of disinfectant

• Presence of organic matter

• pH

• Temperature

• Time

Use-Dilution and Disk-Diffusion Tests

• Use-Dilution Test: Metal cylinders dipped in test bacteria, exposed to disinfectant, then transferred to culture media to check for survival.

• Disk-Diffusion Method: Filter paper disks soaked in chemicals are placed on a culture; zone of inhibition indicates efficacy.

Phenol and Phenolics

• Phenol: First used by Joseph Lister; injures plasma membranes, causing leakage.

• Phenolics: Derivatives of phenol; reduced irritation, increased effectiveness. Example: O-phenylphenol (ingredient in Lysol®).

Bisphenols

• Contain two phenol groups connected by a bridge.

• Disrupt plasma membranes.

• Examples: Hexachlorophene (skin infections), Triclosan (antimicrobial soaps, discontinued except in toothpaste).

Essential Oils (EOs)

• Mixtures of hydrocarbons extracted from plants.

• Microbial action due to phenolics and terpenes.

• Stronger activity against gram-positive bacteria.

• Tea tree oil and pine oil: Broad spectrum activity.

Halogens

• Iodine: Impairs protein synthesis and alters membranes. Used as tincture or iodophor (Betadine®).

• Chlorine: Oxidizing agent; shuts down cellular enzyme systems. Used in bleach (hypochlorous acid), chloramine, and municipal water disinfection.

Alcohols

• Denature proteins and dissolve lipids.

• No effect on endospores and nonenveloped viruses.

• Ethanol and isopropanol: Require water for effectiveness.

• Alcohol-based hand sanitizers: Effective against most bacteria, not against Clostridium difficile or Norovirus.

Heavy Metals and Their Compounds

• Oligodynamic action: Very small amounts exert antimicrobial activity by denaturing proteins.

• Examples: Silver nitrate (prevents ophthalmia neonatorum), silver-impregnated dressings, mercuric chloride (limited use), copper sulfate (algicide), zinc chloride (mouthwash).

Surface-Active Agents

• Soap: Degerming and emulsification.

• Acid-anionic sanitizers: Anions react with plasma membrane; used in food processing.

• Quaternary ammonium compounds (quats): Cations are bactericidal, denature proteins, disrupt plasma membrane. Broad spectrum, but ineffective against endospores and mycobacteria.

Chemical Food Preservatives

• Sulfur dioxide: Prevents wine spoilage.

• Organic acids: Inhibit metabolism; sorbic acid, benzoic acid, calcium propionate prevent molds.

• Nitrites and nitrates: Prevent endospore germination (Clostridium botulinum); preserve red color of meat.

Antibiotics for Food Preservation

• Bacteriocins: Proteins produced by one bacterium that inhibit another. Nisin and natamycin prevent spoilage of cheese.

Aldehydes

• Inactivate proteins by cross-linking with functional groups.

• Formalin: Used for preserving specimens; limited use due to cancer risk.

• Glutaraldehyde: Liquid chemical sterilant; used for medical equipment.

Gaseous Chemosterilants

• Cause alkylation: Cross-link nucleic acids and proteins.

• Ethylene oxide: Used in sealed chambers for sterilizing large equipment.

• Chlorine dioxide: Used for building areas and water treatment.

Plasma and Supercritical Fluids

• Plasma: Electrically excited gas; free radicals destroy microbes. Used for tubular surgical instruments.

• Supercritical fluids: CO2 compressed to a state with both gaseous and liquid properties; used in food industry and medical implants.

Peroxygens and Other Forms of Oxygen

• Peroxygens: Oxidizing agents (hydrogen peroxide, peracetic acid) used for disinfecting surfaces and medical equipment.

• Benzoyl peroxide: Used in topical acne medications.

• Ozone: Used for water disinfection.

Microbial Characteristics and Microbial Control

Microbial resistance to biocides varies by type and structure.

• Gram-negative bacteria: More resistant due to lipopolysaccharide outer membrane.

• Mycobacteria: Considerable resistance; require special testing.

• Bacterial endospores: Very resistant to many biocides.

• Nonenveloped viruses: More resistant than enveloped viruses.

• Prions: Highly resistant; require immersion in NaOH and autoclaving at 121°C for 1 hour.

Effectiveness of Chemical Antimicrobials Against Endospores and Mycobacteria

Chemical Agent	Effect against Endospores	Effect against Mycobacteria
Glutaraldehyde	Fair	Good
Chlorines	Fair	Fair
Alcohols	Poor	Good
Iodine	Poor	Good
Phenolics	Poor	Good
Chlorhexidine	None	Fair
Bisphenols	None	None
Quats	None	None
Silver	None	None

Example: The disk-diffusion method can be used to compare the effectiveness of disinfectants against different types of bacteria, such as Staphylococcus aureus (gram-positive), Escherichia coli (gram-negative), and Pseudomonas aeruginosa (highly resistant gram-negative).

Additional info: These notes expand on the original content by providing definitions, context, and examples for each method and agent, ensuring a comprehensive and self-contained study guide for exam preparation.