The Control of Microbial Growth: Principles and Methods

Chapter 7: 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, principles, 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.

Principles of Microbial Death

Factors Affecting Microbial Death

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

• Environment: Presence of organic matter, temperature, and biofilms can affect effectiveness.

• Time of exposure: Longer exposure increases effectiveness.

• Microbial characteristics: Endospores, cell wall structure, and other features influence resistance.

Actions of Microbial Control Agents

• 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 function.

Physical Methods of Microbial Control

Heat

• Denatures enzymes and other proteins, leading to cell 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 at a given temperature.

Moist Heat Sterilization

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

• Autoclaving: 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 times. Test strips indicate sterility.

Pasteurization and Dry Heat

• Pasteurization: Reduces spoilage organisms and pathogens in liquids. High-temperature short-time (HTST) is 15 seconds; ultra-high-temperature (UHT) is 4 seconds, allowing storage without refrigeration.

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

Filtration

• Used for heat-sensitive materials. Substances pass through a screenlike material (e.g., membrane filters, HEPA filters).

• Membrane filters can remove microbes as small as 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 via singlet oxygen formation.

• 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

Evaluating Disinfectants

• 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 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.

• Halogens: Iodine (impairs protein synthesis, alters membranes), Chlorine (oxidizing agent, used in water disinfection).

• Alcohols: Denature proteins and dissolve lipids; ineffective against endospores and nonenveloped viruses.

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

Other Chemical Methods

• Surface-active agents: Soaps and detergents lower surface tension, aiding in mechanical removal of microbes.

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

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

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

Microbial Characteristics and Resistance

Microbial Resistance to Control Methods

• 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 testing for tuberculocidal activity.

• Bacterial endospores: Highly resistant to many 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	Notes
Autoclaving	All microbes, endospores	High	Preferred for sterilization; not effective against prions
Filtration	Heat-sensitive liquids	High	Removes bacteria, some viruses
Alcohols	Bacteria, enveloped viruses	Moderate	Ineffective against endospores, nonenveloped viruses
Halogens	Bacteria, viruses, fungi	High	Iodine and chlorine commonly used
Heavy metals	Bacteria, fungi, algae	Moderate	Oligodynamic action; toxicity limits use

Key Equations

• Decimal Reduction Time (DRT): $D = \text{time to reduce population by 90\% at a given temperature}$

Conclusion

Understanding the principles and methods of microbial control is crucial for preventing infection, ensuring food safety, and maintaining sterile environments in healthcare and laboratory settings. Both physical and chemical methods have specific applications, advantages, and limitations depending on the type of microbe and the context of use.