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 medical 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: The removal or destruction of all microbial life, including endospores. Methods include heat and filtration. Used for surgical instruments and culture media.

• Disinfection: The destruction of vegetative pathogens on inanimate objects. Does not necessarily kill endospores.

• Antisepsis: Destruction of vegetative pathogens on living tissue. Antiseptics are less toxic than disinfectants.

• Sanitization: Reduces microbial numbers to safe levels, such as on eating utensils.

• Biocide/Germicide: Agents that kill microbes.

• Bacteriostasis: Inhibits bacterial growth without killing.

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

• Nosocomial: Refers to healthcare-associated infections.

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

• The larger the initial population, the longer it takes to achieve sterilization.

• Death rate is influenced by the number of microbes, time of exposure, environmental conditions, and microbial characteristics.

Factors Affecting the Effectiveness of Antimicrobial Treatments

• Number of Microbes: More microbes require longer exposure.

• Time of Exposure: Resistant microbes (e.g., endospores) need longer treatment.

• Environmental Influences: Warm temperatures and low pH enhance effectiveness; organic matter and biofilms inhibit it.

• Microbial Characteristics: Different species have varying susceptibilities.

Actions of Microbial Control Agents

Mechanisms of Action

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

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

• Damage to Nucleic Acids: Prevents replication and normal cell function.

Physical Methods of Microbial Control

Moist Heat Sterilization

• Autoclaving: Uses steam under pressure (121°C, 15 psi, 15 min) to denature proteins and sterilize materials. Steam must contact all surfaces.

• Pasteurization: Reduces microbial load in liquids (e.g., milk) without sterilizing. High-Temperature Short-Time (HTST): 72°C for 15 sec; Ultra-High Temperature (UHT): 140°C for 4 sec (sterilizes).

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

Dry Heat Sterilization

• Methods: Flaming, incineration, and hot-air ovens (170°C for 2 hours).

• Mechanism: Kills by oxidation, damaging DNA, membranes, and proteins.

Filtration

• Removes microbes from liquids and air using physical barriers (HEPA filters for air, membrane filters for fluids).

• Pore sizes: 0.2–0.4 μm for bacteria, 0.01 μm for viruses.

Low Temperature, Desiccation, and Osmotic Pressure

• Low Temperature: Refrigeration slows growth (bacteriostatic); freezing forms ice crystals that damage cells.

• Desiccation: Removes water, preventing metabolism; microbes may 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.

• 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: Lysol, hexachlorophene, triclosan.

• Halogens: Chlorine (forms hypochlorous acid in water, broad spectrum), iodine (tinctures and iodophors, e.g., Betadine).

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

• Heavy Metals: Silver, copper, and zinc denature proteins by binding sulfhydryl groups (oligodynamic action).

• Soaps and Detergents: Mechanical removal of microbes; quaternary ammonium compounds (quats) disrupt membranes, especially effective against Gram-positive bacteria.

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

• Aldehydes: Inactivate proteins by cross-linking; glutaraldehyde is a liquid sterilant for delicate instruments.

• Gaseous Sterilants: Ethylene oxide sterilizes heat-sensitive materials by blocking DNA replication and enzyme activity.

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

Microbial Characteristics and Resistance to Control Agents

• Microbes vary in resistance to control agents. Prions and endospores are most resistant; enveloped viruses are least resistant.

• Gram-negative bacteria are more resistant than Gram-positive due to their outer membrane.

• Naked viruses are more resistant than enveloped viruses because their protein capsid is less susceptible to drying and chemicals.

Summary Table: Microbial Resistance to Chemical Biocides

Most Resistant	Least Resistant
Prions	Viruses with lipid envelopes
Endospores of bacteria	Gram-positive bacteria
Mycobacteria	Fungi, including most fungal spores
Cysts of protozoa	Gram-negative bacteria
Vegetative protozoa	Viruses without envelopes

Conclusion

Effective microbial control requires understanding the mechanisms of action, the physical and chemical methods available, and the characteristics of the target organisms. Selection of appropriate methods depends on the application, the nature of the material to be treated, and the resistance of the microbes present.