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

Chapter 7: The Control of Microbial Growth

7.1: The Terminology of Microbial Control

This section introduces essential terms and concepts related to the control of microbial growth, which are foundational for understanding disinfection, sterilization, and related practices in microbiology.

• Sepsis: Refers to bacterial contamination. Common causative agents include Staphylococcus aureus, Streptococcus pyogenes, Klebsiella spp., Escherichia coli, and Pseudomonas aeruginosa. Sepsis is a life-threatening systemic response to infection.

• Asepsis: The absence of significant contamination. Aseptic techniques are crucial in surgery to prevent microbial contamination of wounds.

• Sterilization: The removal or destruction of all microbial life, including endospores. Commercial sterilization targets Clostridium botulinum endospores in canned goods but may not achieve complete sterilization.

• Disinfection: The destruction of harmful microorganisms, typically on inanimate objects.

• Antisepsis: The destruction of harmful microorganisms on living tissue.

• Degerming: Mechanical removal of microbes from a limited area (e.g., alcohol swab before injection).

• Sanitization: Lowering microbial counts on eating utensils to safe levels (e.g., high-heat dishwasher).

• Biocide (Germicide): Agents that kill microbes; the suffix -cide means to kill.

• Bacteriostasis: Inhibition, not killing, of microbes; the suffix -stat means to inhibit.

• Asepsis: The absence of pathogens from an object or area.

7.2: The Rate of Microbial Death

Microbial populations typically die at a constant rate when exposed to antimicrobial agents. Understanding the death curve is essential for effective sterilization and disinfection.

• Factors Affecting Death Rate:

  • Number of microbes present

  • Environmental factors (organic matter, temperature, biofilms)

  • Time of exposure

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

• Microbial Death Curve: When plotted logarithmically, the death curve is a straight line, indicating a constant percentage of cells die per unit time.

• Key Terms:

  • 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 given temperature.

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

7.3: Actions of Microbial Control Agents

Microbial control agents act by targeting essential cellular components, leading to cell death or inhibition.

• Alteration of Membrane Permeability: Damage to the plasma membrane causes cellular contents to leak, leading to cell death.

• Damage to Proteins (Enzymes): Breaking hydrogen and covalent bonds denatures proteins, inactivating enzymes.

• Damage to Nucleic Acids: Interferes with DNA and RNA, preventing replication and protein synthesis.

7.4: Physical Methods of Microbial Control

Physical methods are widely used to control microbial growth in medical, laboratory, and food industry settings.

Heat

• Moist Heat Sterilization: Coagulates and denatures proteins. Methods include boiling, free-flowing steam, and autoclaving.

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

• Pasteurization: Reduces spoilage organisms and pathogens in food. High-temperature short-time (HTST) pasteurization is 72°C for 15 seconds; ultra-high-temperature (UHT) is 140°C for 4 seconds.

• Dry Heat Sterilization: Kills by oxidation (e.g., flaming, incineration, hot-air sterilization).

• Filtration: Used for heat-sensitive materials. HEPA filters remove microbes >0.3 μm; membrane filters can remove viruses and large proteins.

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

• High Pressure: Denatures proteins; endospores are resistant.

• 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) damages DNA by causing lethal mutations.

  • Nonionizing Radiation: (UV, 260 nm) damages DNA by creating thymine dimers.

  • Microwaves: Kill by heat, not especially antimicrobial.

7.5: Chemical Methods of Microbial Control

Chemical agents are used to disinfect, sanitize, and sterilize surfaces, instruments, and living tissues. Their effectiveness depends on several factors and is evaluated by standardized tests.

Principles of Effective Disinfection

• Concentration of disinfectant

• Presence of organic matter

• pH of the environment

• Time of exposure

Testing Disinfectant Efficacy

• Use-Dilution Test: Measures the effectiveness of a disinfectant against selected microbes.

• Disk-Diffusion Method: Filter paper disks soaked in chemicals are placed on a microbial culture; zones of inhibition indicate effectiveness.

Major Classes of Chemical Agents

• Phenol and Phenolics: Injure plasma membranes, remain active in organic matter, and persist for long periods.

• Bisphenols: Derivatives of phenol with two phenol groups; effective against gram-positive bacteria (e.g., hexachlorophene, triclosan).

• Biguanides: Broad-spectrum, especially effective against gram-positive bacteria (e.g., chlorhexidine).

• Essential Oils: Plant-derived, primarily phenolics and terpenes; more effective against gram-positive bacteria.

• Halogens: Iodine (tincture, iodophor) impairs protein synthesis and alters membranes; chlorine is an oxidizing agent (e.g., bleach).

• Alcohols: Denature proteins and dissolve lipids; ineffective against endospores and nonenveloped viruses. Ethanol and isopropanol require water for activity.

• Heavy Metals: Oligodynamic action; denature proteins (e.g., silver nitrate, mercuric chloride, copper sulfate, zinc chloride).

• Surface-Active Agents: Soaps (degerming), acid-anionic sanitizers, and quaternary ammonium compounds (quats) disrupt membranes and denature proteins.

• Chemical Food Preservatives: Sulfur dioxide, organic acids, nitrites, and nitrates prevent spoilage and endospore germination.

• Antibiotics: Bacteriocins (e.g., nisin, natamycin) prevent spoilage in cheese.

• Aldehydes: Inactivate proteins by cross-linking; used for preserving specimens and sterilizing medical equipment (e.g., glutaraldehyde, formaldehyde).

• Chemical Sterilization: Gaseous sterilants (e.g., ethylene oxide) and plasma sterilizers are used for heat-sensitive materials.

• Supercritical Fluids: Used for sterilizing medical implants; combine properties of gases and liquids.

• Peroxygens: Oxidizing agents (e.g., hydrogen peroxide, peracetic acid) used for surfaces and food packaging.

7.6: Microbial Characteristics and Microbial Control

The effectiveness of chemical antimicrobials varies with the type of microbe. Endospores and mycobacteria are particularly resistant to many agents.

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

Note: Gram-negative bacteria are generally more resistant to chemical biocides than gram-positive bacteria due to their outer membrane structure.

Summary Table: Key Terms in Microbial Control

Term	Definition	Example/Application
Sterilization	Destruction of all microbial life	Autoclaving surgical instruments
Disinfection	Destruction of harmful microbes	Bleach on surfaces
Antisepsis	Destruction of microbes on living tissue	Iodine on skin
Degerming	Mechanical removal of microbes	Alcohol swab before injection
Sanitization	Lowering microbial counts to safe levels	Dishwasher for utensils
Bacteriostasis	Inhibiting, not killing, microbes	Refrigeration