The Control of Microbial Growth

Introduction

The control of microbial growth is a fundamental aspect of microbiology, essential for preventing infection, ensuring food safety, and maintaining sterile environments in healthcare and laboratory settings. This chapter explores the terminology, mechanisms, and methods used to control microbial populations.

Terminology of Microbial Control

Key Terms and Definitions

• Sepsis: Refers to bacterial contamination.

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

• Sterilization: Removal and destruction of all microbial life.

• Commercial Sterilization: Killing Clostridium botulinum endospores in canned goods.

• Disinfection: Destroying harmful microorganisms on inanimate surfaces.

• Antisepsis: Destroying harmful microorganisms from living tissue.

• Degerming: Mechanical removal of microbes from a limited area.

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

• Biocide (Germicide): Treatments that kill microbes.

• Bacteriostasis: Inhibiting, not killing, microbes.

The Rate of Microbial Death

Patterns and Factors Affecting Microbial Death

Microbial death occurs at a logarithmic rate when exposed to control agents. The effectiveness of treatment depends on several factors:

• Number of microbes: Higher populations require longer exposure.

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

• Time of exposure: Longer exposure increases effectiveness.

• Microbial characteristics: Endospores and cell wall composition influence resistance.

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

Actions of Microbial Control Agents

Effects on Cellular Structures

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

• Damage to proteins (enzymes): Denaturation disrupts metabolic functions.

• Damage to nucleic acids: Prevents replication and transcription, leading to cell death.

Physical Methods of Microbial Control

Heat

Heat is a widely used method for microbial control, primarily through denaturation of enzymes.

• 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 particular temperature.

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

Moist Heat Sterilization

• Boiling: Kills most pathogens but not endospores.

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

• Pasteurization: Reduces spoilage organisms and pathogens in milk and juices. High-temperature short-time (HTST) treatment is for 15 seconds; ultra-high-temperature (UHT) treatment sterilizes milk for storage without refrigeration.

Dry Heat Sterilization

• Kills by oxidation: Methods include flaming, incineration, and hot-air sterilization (oven for 2 hours).

Filtration

Filtration is used for heat-sensitive materials, allowing passage of substances through a screenlike material.

• HEPA filters: Remove microbes larger than 0.3 µm.

• Membrane filters: Pore sizes as small as 0.22 µm can filter out 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 by causing lethal mutations.

• Nonionizing radiation: (UV, 260 nm) damages DNA by creating thymine dimers; effective for surfaces but does not penetrate deeply.

• 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

Use-Dilution Tests

Standard method for evaluating disinfectants. Metal cylinders are dipped in test bacteria, dried, placed in disinfectant, then transferred to culture media to determine survival.

Disk-Diffusion Method

Evaluates efficacy of chemical agents by placing filter paper disks soaked in chemicals on a culture and observing zones of inhibition.

Types of Chemical Disinfectants

Phenol and Phenolics

• First used by Joseph Lister to control surgical infections.

• Injure lipids of plasma membranes, causing leakage.

• Remain active in presence of organic matter.

• Phenolics: Derivatives of phenol with reduced irritation and increased effectiveness (e.g., O-phenylphenol in Lysol®).

Bisphenols

• Contain two phenol groups connected by a bridge.

• Disrupt plasma membranes.

• Examples: Hexachlorophene (skin infections), Triclosan (antimicrobial soaps, discontinued except in toothpaste).

Biguanides

• Effective against gram-positive bacteria, many gram-negative bacteria, and enveloped viruses.

• Disrupt plasma membranes.

• Examples: Chlorhexidine (surgical hand scrubs), Alexidine (works faster).

Essential Oils (EOs)

• Mixtures of hydrocarbons extracted from plants (e.g., peppermint, pine, orange oils).

• Microbial action due to phenolics and terpenes.

• Stronger activity against gram-positive bacteria; effectiveness against viruses not well studied.

• Tea tree oil and pine oil: Broad spectrum activity including bacteria and fungi.

Halogens

• Iodine: Impairs protein synthesis and alters membranes. Used as tincture or iodophor (e.g., Betadine®).

• Chlorine: Oxidizing agent; shuts down cellular enzyme systems. Used in bleach (hypochlorous acid), chloramine, and municipal water disinfection.

Alcohols

• Denature proteins and dissolve lipids.

• No effect on endospores and nonenveloped viruses.

• Ethanol and isopropanol require water for effectiveness.

• Alcohol-based hand sanitizers (about 62% alcohol) are effective against most bacteria but not Clostridium difficile or Norovirus.

Heavy Metals and Their Compounds

• Oligodynamic action: Very small amounts exert antimicrobial activity by denaturing proteins.

• Examples: Silver nitrate (prevents ophthalmia neonatorum), silver-impregnated wound dressings, mercuric chloride (limited use), copper sulfate (algicide), zinc chloride (mouthwash).

Surface-Active Agents

• Quaternary ammonium compounds (quats): Disrupt plasma membranes and denature proteins.

• Benzalkonium chloride: Commonly used disinfectant.

Chemical Food Preservatives

• Sulfur dioxide: Prevents wine spoilage.

• Organic acids: Inhibit metabolism (e.g., sorbic acid, benzoic acid, calcium propionate).

• Nitrites and nitrates: Prevent endospore germination (Clostridium botulinum), preserve red color of meat.

Antibiotics for Food Preservation

• Bacteriocins: Proteins produced by one bacterium that inhibit another (e.g., nisin and natamycin prevent cheese spoilage).

Aldehydes

• Inactivate proteins by cross-linking with functional groups.

• Formalin: Used for preserving specimens; limited use due to cancer risk.

• Glutaraldehyde: Liquid chemical sterilizing agent (sporicidal in 3–10 hours); used for medical equipment.

• Ortho-phthalaldehyde (OPA): May replace glutaraldehyde.

Gaseous Chemosterilants

• Cause alkylation: Replace hydrogen atoms with free radicals, cross-linking nucleic acids and proteins.

• Used for heat-sensitive materials: Ethylene oxide (sealed chamber), chlorine dioxide (enclosed areas or water treatment).

Summary Table: Methods of Microbial Control

Additional info: This summary expands on brief points with academic context, definitions, and examples to ensure completeness and clarity for exam preparation.