The Control of Microbial Growth: Principles and Methods

Chapter 7: The Control of Microbial Growth

Introduction

The control of microbial growth is essential in preventing infections, food spoilage, and contamination in healthcare and industrial settings. This chapter explores the principles, methods, and effectiveness of physical and chemical agents used to control microbial populations.

Key Terminology in Microbial Control

Definitions and Applications

• Sterilization: Removal or destruction of all microbial life, including endospores. Commonly achieved by heat, filtration, or chemical agents.

• Disinfection: Destruction of vegetative pathogens on inanimate objects, not necessarily endospores.

• Antisepsis: Destruction of pathogens on living tissue.

• Sanitization: Reduction of microbial counts to safe public health levels (e.g., eating utensils).

• Biocide/Germicide: Agents that kill microbes.

• Bacteriostasis: Inhibition of bacterial growth without killing.

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

• Nosocomial: Healthcare-associated infections.

Term	Definition	Comments
Sterilization	Destruction/removal of all forms of microbial life	Usually by steam under pressure or sterilizing gas
Commercial Sterilization	Sufficient heat to kill endospores of Clostridium botulinum	May not kill all endospores of other thermophilic bacteria
Disinfection	Destruction of vegetative pathogens	May use physical or chemical methods
Antisepsis	Destruction of vegetative pathogens on living tissue	Almost always by chemical antimicrobials
Degerming	Removal of microbes from a limited area	Mostly mechanical removal by alcohol-soaked swab
Sanitization	Lowering microbial counts to safe public health levels	May be done with high-temperature washing or chemical disinfectant

Principles of Microbial Death

Microbial Death Curve

When exposed to antimicrobial agents, microbial populations die at a constant rate, which can be plotted logarithmically as a straight line. The rate is often expressed as a 90% reduction per minute (one log decrease).

• Factors affecting death rate: Number of microbes, time of exposure, environmental conditions (temperature, pH, organic matter), and microbial characteristics (species, endospore formation, etc.).

Actions of Microbial Control Agents

Mechanisms of Action

• Alteration of membrane permeability: Disrupts the plasma membrane, causing leakage of essential cellular contents and cell death.

• Damage to proteins: Denaturation or alteration of protein structure, including enzymes, blocks metabolic functions.

• Damage to nucleic acids: Prevents replication and normal cellular function.

Physical Methods of Microbial Control

Moist Heat Sterilization

Moist heat denatures proteins and is more effective than dry heat. The autoclave uses steam under pressure (121°C at 15 psi for 15 minutes) to achieve sterilization. Steam must contact all surfaces to be effective.

• Limitations: Not suitable for heat-sensitive materials.

• Prion destruction: Requires higher temperature and/or longer exposure with NaOH.

Pressure (psi above atmospheric)	Temperature (°C)
0	100
5	110
10	116
15	121
20	126
30	135

Pasteurization

Pasteurization uses mild heat to reduce microbial numbers in food and beverages, especially pathogens, without significantly affecting taste or quality. High-Temperature Short-Time (HTST) is 72°C for 15 seconds; Ultra-High Temperature (UHT) is 140°C for 4 seconds and achieves sterilization.

• Historical goal: Destroy Mycobacterium bovis to prevent tuberculosis.

• Thermoduric organisms: Survive pasteurization but are not usually pathogenic.

Dry Heat Sterilization

Dry heat kills by oxidation. Methods include flaming, incineration, and hot-air ovens. Used for materials that can withstand high temperatures and cannot be sterilized by moist heat.

Filtration

Filtration physically removes microbes from liquids or air. High-Efficiency Particulate Air (HEPA) filters remove particles >0.3 μm from air; membrane filters with pore sizes of 0.2–0.4 μm are used for bacteria, and 0.01 μm for viruses.

Low Temperature, High Pressure, Desiccation, and Osmotic Pressure

• Low temperature: Refrigeration slows microbial growth (bacteriostatic), but psychrotrophs can still grow.

• High pressure: Denatures proteins and inactivates microbes in some foods.

• Desiccation: Removes water, inhibiting metabolism; microbes can survive but not grow.

• Osmotic pressure: High salt or sugar concentrations cause plasmolysis, inhibiting growth.

Radiation

• Ionizing radiation (X-rays, gamma rays): Produces free radicals that damage DNA; used for sterilizing heat-sensitive materials and food.

• Nonionizing radiation (UV): Causes thymine dimers in DNA, inhibiting replication; used for surface and air disinfection.

Chemical Methods of Microbial Control

Evaluating Disinfectants

• Use-Dilution Test: Determines effectiveness of disinfectants against selected microbes.

• Disk-Diffusion Method: Chemical-soaked disks placed on inoculated agar; zone of inhibition indicates effectiveness.

Types of Chemical Disinfectants

• Phenols and Derivatives: Disrupt plasma membranes and denature proteins. Examples: phenol, phenolics (Lysol), bisphenols (hexachlorophene, triclosan).

• Halogens: Chlorine (forms hypochlorous acid in water, broad-spectrum disinfectant), iodine (tinctures and iodophors, used as antiseptics and disinfectants).

• Alcohols: Ethanol and isopropanol denature proteins and dissolve lipids; effective against bacteria, enveloped viruses, and fungi, but not endospores or nonenveloped viruses.

• Heavy Metals: Oligodynamic action; silver nitrate (antiseptic for newborn eyes), copper (algicide), zinc (mouthwashes, antifungal paints).

• Soaps and Detergents: Mechanical removal of microbes; not true disinfectants but aid in cleaning and allow disinfectants to contact microbes.

• Quaternary Ammonium Compounds (Quats): Cationic detergents, strongly bactericidal against Gram-positive bacteria; disrupt plasma membranes but do not kill endospores or mycobacteria.

• Organic Acids and Preservatives: Inhibit metabolism; used in foods, beverages, and cosmetics (e.g., sorbic acid, benzoic acid, sulfur dioxide).

• Aldehydes: Inactivate proteins by cross-linking; glutaraldehyde is a liquid sterilant for delicate instruments, formaldehyde used for embalming and vaccine preparation.

• Gaseous Sterilants: Ethylene oxide is a highly penetrating gas used for sterilizing medical equipment; blocks DNA replication and enzymatic actions.

• Oxidizing Agents: Hydrogen peroxide is effective against anaerobes and used for disinfecting surfaces and packaging.

Microbial Characteristics and Resistance to Control Agents

Relative Resistance of Microbes

Microbial resistance to control agents varies by type. Prions and endospores are most resistant, while enveloped viruses are least resistant. Gram-negative bacteria are more resistant than Gram-positive due to their outer membrane.

Enveloped vs. Naked Viruses

• Enveloped viruses: More susceptible to lipid-dissolving agents (e.g., alcohols, quats) because disruption of the envelope inactivates the virus.

• Naked viruses: More resistant to drying, acids, and detergents; can survive longer on surfaces and are more likely to be transmitted via contaminated materials.

Summary Table: Effectiveness of Physical and Chemical Methods

Method	Effectiveness	Preferred Use
Autoclaving	Highly effective, sterilizes	Media, labware, surgical instruments
Pasteurization	Reduces pathogens, not sterilizing	Milk, juices, eggs
Filtration	Removes microbes	Heat-sensitive liquids, air
Radiation	Sterilizes (ionizing), disinfects (UV)	Medical supplies, food, surfaces
Alcohols	Disinfects, not sporicidal	Skin, surfaces
Halogens	Disinfects, antiseptic	Water, skin, surfaces
Quats	Disinfects, not sporicidal	Surfaces, equipment
Glutaraldehyde	Sterilizes with prolonged exposure	Delicate instruments
Ethylene oxide	Sterilizes	Plastic labware, medical equipment

Key Points for Exam Preparation

• Understand the definitions and differences between sterilization, disinfection, antisepsis, and sanitization.

• Be able to explain the microbial death curve and factors influencing microbial control.

• Compare and contrast physical and chemical methods of microbial control, including their mechanisms and applications.

• Know the resistance hierarchy of microbes and why certain types are more difficult to control.

• Be familiar with the disk-diffusion method and interpretation of results.