Chapter 7: The Control of Microbial Growth – Study Notes

The Control of Microbial Growth

Introduction

The control of microbial growth is essential in healthcare, food safety, and laboratory settings. This chapter explores the terminology, mechanisms, and methods used to inhibit or eliminate microorganisms, including both physical and chemical approaches.

Terminology of Microbial Control

Key Terms and Definitions

• Sterilization: The removal or destruction of all microbial life, including endospores. Commercial sterilization refers specifically to killing Clostridium botulinum endospores in canned goods.

• Disinfection: The destruction of harmful microorganisms on inanimate surfaces or environments.

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

• Degerming: The mechanical removal of microbes from a limited area (e.g., skin before injection).

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

• Biocide (germicide): Treatments that kill microbes.

• Bacteriostasis: Inhibiting, not killing, microbes.

• Sepsis: Refers to bacterial contamination.

• Asepsis: The absence of significant contamination; aseptic techniques prevent microbial contamination of wounds.

Microbial Death

Factors Affecting Microbial Death

• Number of microbes: Larger populations take longer to eliminate.

• Environment: Organic matter, temperature, and biofilms can protect microbes.

• Time of exposure: Longer exposure increases effectiveness.

• Microbial characteristics: Endospores, cell wall structure, and other traits affect resistance.

Actions of Microbial Control Agents

Mechanisms of Action

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

• Damage to proteins (enzymes): Denaturation or inactivation of essential enzymes.

• Damage to nucleic acids: Prevents replication and function.

Physical Methods of Microbial Control

Heat

• Heat denatures enzymes, leading to microbial death.

• 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): Minutes to kill 90% of a specific population of bacteria at a given temperature.

Moist Heat Sterilization

• Boiling and free-flowing steam: Coagulate and denature proteins.

• Autoclave: Steam under pressure (15 psi, 15 min) kills all organisms (except prions) and endospores. Steam must contact the item’s surface. Large containers require longer sterilization times. Test strips are used to indicate sterility.

• Sterilization indicators: Strips that change color to confirm sterilization.

• Pasteurization: Reduces spoilage organisms and pathogens in milk and juices. High-temperature short-time (HTST) is 72°C for 15 sec; ultra-high-temperature (UHT) is 140°C for 4 sec, allowing storage without refrigeration.

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

Filtration

• Passage of a substance through a screenlike material; used for heat-sensitive materials.

• High-efficiency particulate air (HEPA) filters remove microbes >0.3 μm.

• Membrane filters remove microbes as small as 0.22 μm, including some 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 and causing lethal mutations. Used for sterilizing pharmaceuticals, medical supplies, and food.

• Nonionizing radiation: (UV, 260 nm) Damages DNA by creating thymine dimers. Used for surface sterilization (e.g., hospital rooms).

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

• Microwaves: Kill by heat, not especially antimicrobial.

Chemical Methods of Microbial Control

Principles of Effective Disinfection

• Concentration of disinfectant

• Presence of organic matter

• pH

• Temperature

• Time of exposure

Testing Disinfectant Efficacy

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

Major Types of Chemical Agents

• Phenol and Phenolics: Disrupt plasma membranes; remain active in presence of organic matter. Example: O-phenylphenol (Lysol®).

• Biguanides: Disrupt plasma membranes; effective against gram-positive, many gram-negative bacteria, and enveloped viruses. Example: Chlorhexidine.

• Essential Oils: Plant-derived, broad-spectrum activity, especially against gram-positive bacteria. Example: Tea tree oil.

• Halogens: Iodine (impairs protein synthesis, alters membranes; used as tincture or iodophor), Chlorine (oxidizing agent; used in bleach and water disinfection).

• Alcohols: Denature proteins and dissolve lipids; ineffective against endospores and nonenveloped viruses. Example: Ethanol, isopropanol.

• Heavy Metals: Oligodynamic action; denature proteins. Examples: Silver nitrate (prevents ophthalmia neonatorum), copper sulfate (algicide), zinc chloride (mouthwash).

• Surface-active agents: Soaps and detergents; mechanically remove microbes.

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

• Supercritical fluids: Used for sterilizing medical implants and food.

• Antibiotics (for food preservation): Bacteriocins such as nisin and natamycin prevent spoilage in cheese.

Microbial Characteristics and Microbial Control

Microbial Resistance

• Gram-negative bacteria: More resistant to biocides due to their outer membrane's lipopolysaccharide layer.

• Pseudomonas and Burkholderia: Notably resistant to many disinfectants.

• Mycobacteria: Resistant due to waxy cell wall; require special disinfectants (tuberculocides).

• Bacterial endospores: Highly resistant to most biocides.

• Nonenveloped viruses: More resistant than enveloped viruses.

• Prions: Extremely resistant; require immersion in NaOH and autoclaving for 1 hour.

Summary Table: Effectiveness of Physical and Chemical Methods

Method	Target	Effectiveness
Autoclaving	All microbes, endospores (not prions)	Very high
Pasteurization	Pathogens, spoilage organisms	Moderate
Filtration	Heat-sensitive liquids	High (depends on pore size)
Radiation (ionizing)	All cells, spores	Very high
Alcohols	Bacteria, enveloped viruses	Moderate
Halogens	Bacteria, viruses, fungi	High
Heavy metals	Bacteria, fungi, algae	Moderate

Key Takeaways

• Microbial control is achieved through a combination of physical and chemical methods, each with specific applications and limitations.

• Understanding microbial characteristics is essential for selecting effective control strategies.

• Proper sterilization and disinfection are critical in healthcare, food safety, and laboratory environments.