Microbiology Chapter 11: Microbial Control Methods

Microbiology: A Systems Approach, 2nd ed.

• Chapter 11 covers Physical and Chemical Control of Microbes.

Controlling Microorganisms

• Distinguishing between sterilization, disinfection, antisepsis, and decontamination is crucial.
• Sterilization destroys all microbial life.
• Disinfection destroys most microbial life, reducing contamination on inanimate surfaces.
• Antisepsis destroys most microbial life, reducing contamination on living surfaces.
• Decontamination is the mechanical removal of most microbes from inanimate or animate surfaces.

Microbial Control Methods

• Physical agents include heat (dry and moist), radiation (ionizing and nonionizing), and filtration.
• Chemical agents include various compounds, categorized by their mechanism of action.
• Mechanical methods include filtration, removing microbes

Relative Resistance of Microbial Forms

• Endospores are the most resistant microbial forms, followed by other forms.
• The goal for bacterial vegetative cells and endospores is sterilization.
• The resistance of different microbial forms varies, impacting the control methods needed.

Terminology and Methods of Microbial Control

• Sterilization removes all viable microorganisms, including viruses.
• Sterile materials are usually inanimate objects, often relying on heat.

Disinfection

• Disinfection uses a physical method or chemical agent to destroy vegetative pathogens.
• Disinfection does not kill endospores.
• It's used on inanimate objects and also removes toxins.

Antisepsis

• Antiseptics are used directly on exposed body surfaces.
• Sepsis is the growth of microorganisms in the blood and other tissues.
• Asepsis is any practice that prevents infectious agents from entering sterile tissues.

The Agents Versus the Processes

• Germicides kill microorganisms.

• Bactericide kills bacteria.

• Fungicide kills fungal spores, hyphae, and yeasts.

• Virucide inactivates viruses.

• Sporicides kill endospores.

• Germicide and microbicide kill microorganisms.

• Stasis is preventing the growth of microorganisms.

• Bacteristatic prevents bacteria growth.

• Fungistatic inhibits fungal growth

• Microbistatic materials control microorganisms in the body.

Decontamination

• Decontamination reduces the risk of infection or spoilage (e.g. food industry).
• Sanitization mechanically removes microorganisms to reduce contamination to safe levels.
• Sanitizer is usually a compound like soap or detergent.
• Sanitation does not always ensure that no microbes remain but is safe for normal use.
• Degermation reduces the number of microbes on human skin (e.g., alcohol wipes).

Practical Concerns in Microbial Control

• Sterilization or disinfection may be needed based on the application.
• Reusability of items should be considered, along with their ability to withstand processes like heat, pressure, radiation, or chemicals.
• Controlling factors such as application suitability, agent penetration, cost- and labor-effectiveness, and safety are all important.

What is Microbial Death?

• Microbial death occurs when cell structures become dysfunctional and irreversible damage ensues.
• If a cell cannot reproduce under ideal conditions, it's considered dead.

Factors That Affect Death Rate

• The length of exposure, load of microbes, relative resistance of spores versus vegetative forms and the action of the agent are all factors impacting death rates.

Factors That Influence the Action of Antimicrobial Agents

• The number of microorganisms, the nature of the microorganisms, the temperature and pH of the environment, the concentration of the agent, the mode of action of the agent, the presence of solvents, interfering organic matter, and inhibitors.

How Antimicrobial Agents Work: Their Modes of Action

• Agents can affect cell walls (blocking synthesis, digesting or breaking down the surface, leading to cell lysis).
• Cell membrane disruptions are possible with agents.
• Agents can affect cellular synthetic processes and proteins.

Methods of Physical Control

• Six methods of physical control are named and described, along with examples of each method's use.
• Moist and dry heat methods are compared and contrasted.

Heat as an Agent of Microbial Control

• Elevated temperatures are microbicidal.
• Lower temperatures are microbistatic.
• Moist and dry heat can both be used.

Comparison of Times and Temperatures to Achieve Sterilization with Moist and Dry Heat

• Providing a table of temperatures and times for moist and dry heat methods of microbial control

Practical Concerns in the Use of Heat: Thermal Death Measurements

• Temperature and exposure length must be considered.
• Higher temperatures generally allow for shorter times.
• Lower temperatures require longer exposure times.
• Thermal death time (TDT): the shortest time to kill all microbes at a given temperature.
• Thermal death point (TDP): the lowest temperature needed to kill all microbes in a sample in 10 minutes.

Common Methods of Moist Heat Control

• Methods of moist heat control are detailed (steam under pressure, non-pressurized steam, pasteurization, and boiling water).

Steam under pressure

• Pressure raises the temperature of steam allowing for a highly effective sterilization method (Autoclave).
• 15 psi yields 121°C.

Nonpressurized Steam

• Also known as intermittent sterilization or tyndallization, using intermittent heating cycles with incubation periods between exposures.

Pasteurization

• Used to disinfect beverages and fluids.
• Special heat exchangers used for both flash and batch methods.
• Does not kill endospores or thermoduric microbes.

Boiling Water

• Used for disinfection, not sterilization.
• Expose materials to boiling water for 30 minutes.

Dry Heat: Hot Air and Incineration

• Incineration reduces microbes to ashes and gas.
• Burning inoculating loops and needles is a common practice.
• Tabletop infrared incinerators are also used.

Dry Oven

• Electric oven where coils radiate heat within an enclosed compartment.
• Exposure to 150-180°C for 2-4 hours.
• Often used with heat-resistant items that don't sterilize well by moist heat method.

The Effects of Cold and Desiccation

• Cold slows growth but does not kill most microbes; freezing preserves cultures.
• Desiccation dehydrates vegetative cells with direct exposure to room air.
• Lyophilization (freeze-drying) preserves microorganisms for many years.

Radiation as a Microbial Control Agent

• Radiation is high-velocity energy from atomic activities.
• Gamma rays, X rays, and UV radiation are used for microbial control.

Modes of Action of Ionizing Versus Nonionizing Radiation

• Ionizing radiation ejects orbital electrons, forming ions and causing damage.
• Nonionizing radiation excites atoms to higher states but does not ionize them.

Ionizing Radiation: Gamma Rays, X Rays, and Cathode Rays

• Cold sterilization using gamma rays, X rays, and cathode rays.
• Dosage is measured in Grays.
• Exposure ranges from 5 to 50 kiloGrays.
• Penetration power differs between types

Applications of Ionizing Radiation

• Used in food and medical products.

Nonionizing Radiation: Ultraviolet Rays

• Wavelength approximately 100-400 nm.
• Germicidal lamp using 254 nm radiation.
• Not as penetrating as ionizing radiation.

Applications of Ultraviolet Radiation

• Disinfects hospital rooms, operating rooms, schools, food prep areas, and dental offices.
• Used to treat drinking water or purify liquids (a type of disinfection).

Decontamination by Filtration

• Filtering fluids removes microbes with small openings (pores).
• Cellulose acetate, polycarbonate, and plastic types are commonly used.

Applications of Filtration

• Used for liquids that can't withstand heat.
• Decontaminates beverages.
• Purifies water, removes airborne contaminants (HEPA filters).

Chemical Agents in Microbial Control

• Approximately 10,000 antimicrobial chemical agents are manufactured.
• Approximately 1,000 are routinely used (usually in liquid, gaseous, or solid state).

Choosing a Microbicidal Chemical

• Choosing a chemical agent depends on characteristics such as rapid action, stability, broad spectrum, non-toxicity, penetration, resistance to inactivation, non-corrosion, sanitizing/deodorizing properties and affordability.

Factors That Affect the Germicidal Activity of Chemicals

• The nature of microorganisms, the nature of the material, the degree of contamination, the time of exposure, and the strength of the germicide.

Required Concentrations and Times for Chemical Destruction of Selected Microbes

• Table with required concentrations and times for the disinfection/sterilization of specified microbes.

Qualities of Chemical Agents Used in Health Care

• Table providing qualities of chemical agents used in health care, like target microbes, level of activity, toxicity, and comments.

Germicidal Categories According to Chemical Group

• Halogens, heavy metals, alcohols, phenolics compounds, oxidizers, aldehydes, detergents, and gases are chemical groups categorized by their germicidal properties.

Halogen Antimicrobial Chemicals

• Fluorine, bromine, chlorine, and iodine are the halogen microbial chemicals.
• Chlorine forms are microbicidal and sporicidal with longer exposure times.

Phenol and its Derivatives

• Phenol coefficient compares a chemical's antimicrobial properties to those of phenol.
• High concentrations are cellular poisons, and lower concentrations inactivate critical enzyme systems.

Chlorhexidine

• A chlorinated complex organic base used to target cell membranes and protein structure.
• Bactericidal for most bacteria types other than spores.
• It has modest toxicity, is mild and acts quickly.

Alcohols as Antimicrobial Agents

• Ethyl and isopropyl alcohols are common antimicrobial agents, working by dissolving lipids, disrupting surface tension, compromising membranes and through coagulation, dehydrating cells, and inhibiting growth.
• More effective at inactivating enveloped viruses than nonenveloped viruses.

Hydrogen Peroxide and Related Germicides

• Germicidal effects are due to the direct and indirect actions of oxygen, forming hydroxyl free radicals.
• These are highly toxic and reactive to microbes and are used when higher concentrations are needed.

Chemicals with Surface Action: Detergents

• Surfactants, like detergents (anionic (-) and cationic (+)), have varying microbial action powers.
• Cationic detergents are more effective by binding with bacterial surface proteins (due to the positive charge).
• Soaps are weak microbicides, but their germicidal value increases when mixed with substances such as chlorhexidine or iodine.

Heavy Metal Compounds

• Mercury, silver, gold, copper, arsenic, and zinc have been used.
• Hg and Ag are significant germicides.
• They act by binding to functional groups of proteins, inactivating them.
• Drawbacks include toxicity to humans, allergic reactions, neutralization by biological fluids and wastes, and possible bacterial resistance development.

Aldehydes as Germicides

• Aldehyde chemicals contain an -CHO functional group and include glutaraldehyde and formaldehyde (formalin).
• Used for sterilization and high-level disinfection.

Gaseous Sterilants and Disinfectants

• Ethylene oxide (ETO) and propylene oxide, and Chlorine dioxide are important gaseous sterilants and disinfectants.

Dyes as Antimicrobial Agents

• Aniline dyes (crystal violet and malachite green) and Yellow acridine dyes (acriflavine and proflavine) are active against certain bacteria and fungi, but have limitations like staining and a narrow spectrum of activity.

Acids and Alkalis

• Very low or high pH can damage or inhibit bacterial cells.
• Limited by their corrosive, caustic, and hazardous nature.

Active Ingredients of Various Commercial Antimicrobial Products

• Table listing various commercial antimicrobial products with their active chemical ingredients and antimicrobial categories.

Explore the essential methods for controlling microorganisms as detailed in Chapter 11 of 'Microbiology: A Systems Approach'. This quiz covers sterilization, disinfection, antisepsis, and the various physical and chemical agents used for microbial control. Test your understanding of microbial resistance and the effectiveness of different control techniques.