Chapter 7 B ChapSterilization and Disinfection

Fundamental Concepts of Sterilization and Disinfection

• Sterilization: The process of killing or removing all microorganisms in a material or on an object.

• Sterility: A state in which there are no living organisms in or on a material.

  • In laboratory standards, a sample is designated as sterile if the probability of finding a live organism is no greater than one chance in $1,000,000$ ($10^6$).

• Disinfection: This involves reducing the number of pathogenic organisms on objects or in materials to a level where they pose no threat of disease.

• Disinfectants: Chemical agents specifically used on non-living (inanimate) objects, such as tables.

• Antiseptics: Chemical agents that are applied to living tissue.

• Usage Caution: Most disinfectants are too harsh for application on human skin.

Microbial Growth Control and Death Rates

• Heat as an Antimicrobial Agent: Heat is an effective agent; however, the death rate of organisms remains logarithmic when heat is applied.

• Logarithmic Death Rate Examples:

  • If heat is applied and $20\%$ of the organisms die in the first minute, then $20\%$ of the remaining organisms will die in the second minute.

  • At a different (presumably higher) temperature, if $30\%$ die in the first minute, $30\%$ of those remaining will die in the second minute.

  • The application of heat greatly accelerates this logarithmic death rate compared to natural conditions.

Comprehensive Terminology of Antimicrobial Control

• Sterilization: Killing or removal of all microorganisms in a material or on an object.

• Disinfection: Reduction of the number of pathogenic microorganisms to the point where they pose no danger of disease.

• Antiseptic: A chemical agent that can safely be used externally on living tissue to destroy microorganisms or inhibit their growth.

• Disinfectant: A chemical agent used on inanimate objects to destroy microorganisms. Most do not kill spores.

• Sanitizer: A chemical agent typically used on food-handling equipment and utensils to reduce bacterial numbers to meet public health standards. This may involve thorough washing with only soap or detergent.

• Bacteriostatic Agent: An agent that inhibits the growth of bacteria but does not necessarily kill them.

• Germicide: An agent capable of killing microbes rapidly; some may kill certain microorganisms while only inhibiting others.

• Bactericide: An agent that kills bacteria. Most bactericides do not kill spores.

• Viricide: An agent that inactivates viruses.

• Fungicide: An agent that kills fungi.

• Sporocide: An agent that kills bacterial endospores or fungal spores.

Factors Influencing the Potency of Chemical Agents

• Primary Factors: The effectiveness (potency) of a chemical antimicrobial agent is affected by time, temperature, pH, and concentration.

• Time: The length of exposure directly affects the death rate of organisms.

• Concentration:

  • High concentrations may be bactericidal (killing).

  • Lower concentrations may be bacteriostatic (growth-inhibiting).

• The Alcohol Paradox:

  • Isopropanol was long believed to be more potent at $70\%$ concentration than at higher concentrations (though it is still effective at $99\%$).

  • The Role of Water: Water is mandatory for alcohols to disinfect because they act by coagulating (permanently denaturing) proteins. Water is a necessary component for the coagulation reaction to occur.

  • Penetration: A $70\%$ alcohol-water mixture penetrates materials more deeply than pure alcohol.

Evaluation Standards: The Phenol Coefficient

• History: In $1867$, Joseph Lister introduced phenol (carbolic acid) as a disinfectant.

• The Standard: Since then, phenol has served as the baseline standard against which other disinfectants are compared under identical conditions.

• Definition: The result of this comparison process is known as the Phenol Coefficient.

Criteria for Selecting the Ideal Disinfectant

In practice, professionals choose the agent that meets the most criteria for a specific task. An ideal disinfectant should:

1. Be fast-acting, even when organic substances (like body fluids) are present.

2. Be effective against all types of infectious agents without destroying tissues or being poisonous if ingested.

3. Easily penetrate the material without causing damage or discoloration.

4. Be easy to prepare and remain stable when exposed to light, heat, or environmental factors.

5. Be inexpensive and easy to obtain/use.

6. Not possess an unpleasant odor.

Mechanisms of Action: Molecular Targets

Reactions Affecting Proteins

• Denaturation: The alteration of protein structure. Denatured proteins lose their functional capacity.

• Temporary Denaturation: Some proteins are temporarily denatured by mild heat, diluted acids, or alkalis. They may regain normal structure once the agent is removed (e.g., warmed milk).

• Permanent Denaturation: Most antimicrobial agents are strong enough to cause permanent denaturation, which kills the microorganism (e.g., a fried egg).

Reactions Affecting Membranes

• Composition: Because membranes contain proteins and lipids, agents that affect these components disrupt cell integrity.

• Surfactants: These are soluble compounds that reduce surface tension (similar to how soap breaks up grease). Examples include alcohols, detergents, and quaternary ammonium compounds which dissolve lipids.

• Phenols: These act by both dissolving lipids and denaturing proteins.

• Detergents: While soaps and detergents may not kill microorganisms directly, they help other agents penetrate fatty substances.

Reactions Affecting Other Components

• Nucleic Acids: Alkylating agents can replace hydrogen on amino or alcohol groups in nucleic acids, disrupting genetic information.

• Cell Walls: Certain dyes, such as crystal violet, interfere with the formation of the cell wall.

• Energy Systems: End products of fermentation, such as lactic acid and propionic acid, can inhibit the fermentation process and prevent energy production in bacteria and molds.

Reactions Affecting Viruses

• Inactivation: Viruses must be inactivated (rendered permanently incapable of infecting or replicating).

• Methods: Viruses are inactivated by destroying their nucleic acids or their proteins.

• Complexity: Viruses can sometimes remain infective even after protein denaturation. Traditional bacterial control methods may not be as successful against infectious viruses, posing a risk for laboratory-acquired infections.

Chemical Antimicrobial Agents: Detailed Properties

• Soaps and Detergents: Lower surface tension; used for hand washing, laundering, and sanitizing equipment.

• Surfactants: Dissolve lipids and disrupt membranes. Cationic detergents sanitize utensils; anionic detergents are used for laundering; quaternary ammonium compounds are skin antiseptics.

• Acids: Lower pH and denature proteins; used in food preservation.

• Alkalis: Raise pH and denature proteins; found in soaps.

• Heavy Metals: Denature proteins.

  • Silver nitrate: Prevents gonococcal infections.

  • Mercury compounds: Disinfect skin and objects.

  • Copper: Inhibits algal growth.

  • Selenium: Inhibits fungal growth.

• Halogens: Oxidize cell components. Chlorine kills waterborne pathogens; iodine is a skin antiseptic.

• Alcohols: Denature proteins when mixed with water. Isopropyl alcohol for skin; ethylene/propylene glycol for aerosols.

• Phenols: Disrupt membranes and denature proteins; not impaired by organic matter. Chlorhexidine gluconate is used as a surgical scrub.

• Oxidizing Agents: Disrupt disulfide bonds. Hydrogen peroxide cleans puncture wounds; potassium permanganate disinfects instruments.

• Alkylating Agents: Disrupt protein and nucleic acid structures. Formaldehyde inactivates viruses for vaccines; glutaraldehyde sterilizes equipment; ethylene oxide sterilizes heat-sensitive inanimate objects.

• Dyes: Interfere with replication or cell wall synthesis. Acridine cleans wounds; crystal violet treats protozoan/fungal infections.

Physical Antimicrobial Agents: Detailed Properties

• Dry Heat: Denatures proteins. Used in ovens for glassware/metal; open flames for incineration.

• Moist Heat: Denatures proteins. Autoclaving sterilizes media and equipment; pressure cooking for canned foods.

• Pasteurization: Denatures proteins. Kills pathogens in milk, dairy, and beer.

• Refrigeration: Slows enzyme-controlled reactions. Keeps food fresh for days; does not kill most microbes.

• Freezing: Greatly slows enzyme reactions. Keeps food for months; used with glycerol to preserve microorganisms.

• Drying: Inhibits enzymes. Preserves fruits, vegetables, and meats.

• Freeze-drying (Lyophilization): Dehydration inhibits enzymes. Used for instant coffee and long-term microbe preservation.

• Ultraviolet (UV) Light: Denatures proteins and nucleic acids. Reduces microbes in the air of operating rooms and transfer areas.

• Ionizing Radiation: Denatures proteins and nucleic acids. Sterilizes plastics, pharmaceuticals, and preserves food.

• Microwave Radiation: Absorbs water molecules then releases energy as heat. Not reliably used for microbes except in specialized equipment.

• Strong Visible Light: Oxidation of light-sensitive materials. Can sanitize clothing when used with dyes.

• Sonic/Ultrasonic Waves: Cause cavitation. Used for studying/fractionating cell components rather than practical killing.

• Filtration Membranes: Mechanically removes microbes. Used for heat-sensitive media, vitamins, vaccines, and air/water sampling.

• Osmotic Pressure: Removes water from microbes. Prevents spoilage in pickles and jellies.

Advanced Sterilization Procedures

Autoclaving

• Mechanism: A machine that applies steam heat under pressure.

• Applications: Used for microbiological media, medical equipment, and laboratory tools.

• Standard Settings:

  • Pressure: $15\,pounds/in^2$ ($15\,psi$).

  • Temperature: $121^\circ C$ ($294^\circ F$).

  • Duration: Most substances are sterile within $20\,minutes$.

Filtration Pore Sizes and Particle Retention

• $10\,\mu m$: Allows erythrocytes, yeast, bacteria, and viruses to pass.

• $5\,\mu m$: Allows yeast, bacteria, and viruses to pass.

• $3\,\mu m$: Allows some yeast, bacteria, and viruses to pass.

• $1.2\,\mu m$: Allows most bacteria and viruses to pass.

• $0.45\,\mu m$: Allows a few bacteria and all viruses to pass.

• $0.22\,\mu m$: Allows viruses and molecules to pass.

• $0.10\,\mu m$: Allows medium to small viruses to pass.

• $0.05\,\mu m$: Allows small viruses to pass.

• $0.025\,\mu m$: Allows only the very smallest viruses to pass.

• Ultrafilter: Allows only small molecules to pass.