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Control of Microbial Growth
Chapter 5
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Approaches to Control Control mechanisms either physical or chemical
May be a combination of both Physical methods Heat Irradiation Filtration Mechanical removal Chemical methods Use a variety of antimicrobial chemicals Chemical depends on circumstances and degree of control required
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Approaches to Control Principles of control Sterilization Disinfection
Removal of all microorganisms Sterile item is absolutely free of microbes, endospores and viruses Can be achieved through filtration, heat, chemicals and irradiation Disinfection Eliminates most pathogens Some viable microbes may exist Disinfectants = used on inanimate objects and surfaces Antiseptics = used on living tissues Pasteurization Brief heat treatment used to reduce organisms that cause food spoilage Surfaces can also be pasteurized
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Approaches to Control Principles of control Decontamination Degerming
Treatment to reduce pathogens to level considered safe Degerming Mechanism uses to decrease number of microbes in an area Particularly the skin Sanitized Implies a substantially reduced microbial population This is not a specific level of control Preservation Process used to delay spoilage of perishable items Often includes the addition of growth-inhibiting ingredients
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Approaches to Control Situational considerations
Microbial control methods are highly variable Depends on situation and degree of control required Daily life Hospital Microbiology laboratories Food and food production facilities Water treatment
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Approaches to Control Daily life
Washing a scrubbing with soaps and detergents achieves routing control Hand washing single most important step to achieving control Soap acts as wetting agent Aids in mechanical removal of microorganisms Removes numerous organism from outer layer of skin Normal flora usually unaffected because it resides in deeper layers
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Approaches to Control Hospitals
Minimizing microbial population very important Due to danger of nosocomial infections Patients are more susceptible to infection Pathogens more likely found in hospital setting Numerous organisms develop antimicrobial resistance due to high concentrations of antibiotics Instruments must be sterilized to avoid introducing infection to deep tissues
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Approaches to Control Microbiology laboratories
Use rigorous methods of control To eliminate microbial contamination to experimental samples and environment Aseptic technique and sterile media used for growth Eliminates unwanted organisms Contaminated material treated for disposal Eliminate contamination of environment
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Approaches to Control Food and food production facilities
Retention of food quality; enhanced through prevention of microbial growth and contamination Achieved through physical removal and chemical destroying organisms Heat treatment most common and most reliable mechanism Irradiation approved to treat certain foods Chemicals prevent spoilage Risk of toxicity
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Approaches to Control Water treatment facilities
Ensures drinking water is safe Chlorine generally used to disinfect water Can react with naturally occurring chemicals Form disinfection by-products (DBP) Some DBP linked to long-term health risks Some organism resistant to chemical disinfectants
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Selection of Antimicrobial Procedure
Selection of effective procedure is complicated Ideal method does not exist Each has drawbacks and procedural parameters Choice of procedure depends on numerous factors Type of microbe Extent of contamination Number of organisms Environment Risk of infection Composition of infected item
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Selection of Antimicrobial Procedure
Type of microorganism Most critical consideration Is organism resistant or susceptible to generally accepted methods? Resistant microbes include Bacterial endospores Resistant to heat, drying and numerous chemicals Protozoan cysts and oocysts Generally excreted in feces and cause diarrheal disease Mycobacterium species Cell wall structure initiates resistance Pseudomonas species Can grow in presence of many chemical disinfectants Naked viruses Lack envelope and are more resistant to chemical killing
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Selection of Antimicrobial Procedure
Number of organisms initially present Time it takes to kill it directly affected by population size Large population = more time Commercial effectiveness is gauged by decimal reduction time A.k.a D value Time required to kill 90% of population under specific conditions Washing reduces time required to reach disinfection or sterilization
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Selection of Antimicrobial Procedure
Environmental conditions Environmental conditions strongly influence effectiveness pH, temperature and presence of organic materials can increase or decrease effectiveness Most chemicals are more effective at higher temperatures and lower pH Effectiveness can be hampered by the presence of organism molecules Can interfere with penetration of antimicrobial agent
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Selection of Antimicrobial Procedure
Potential risk of infection Medical items categorized according to potential risk of disease transmission Critical items = come in contact with body tissues Needles and scalpels Semicritical instruments = contact mucous membranes but do not penetrate body tissues Endoscope Non-critical instruments = contact unbroken skin only Show little risk of transmission Stethoscope
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Selection of Antimicrobial Procedure
Composition of the item Some sterilization and disinfection methods inappropriate for certain items Heat inappropriate for plastics and other heat sensitive items
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Heat as Control Heat treatment most useful for microbial control
Relatively fast, reliable, safe and inexpensive Heat can be used to sterilize or disinfect Methods include Moist heat Dry heat
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Heat as Control Moist heat
Destroys through irreversible coagulation of proteins Moist heat includes Boiling Pasteurization Pressurized steam
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Heat as Control Boiling (100 C) Pasteurization
Destroys most microorganisms and viruses Not effective means of sterilization Does not destroy endospores Pasteurization Pasteur developed to avoid spoilage of wine Does not sterilize but significantly reduces organisms Used to increase shelf life of food Most protocols employ HTST method Heated to 72°C and held for 15 seconds Other protocol UHT Heated to 140°C - 150°C, held for several seconds then rapidly cooled
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Heat as Control Pressurized steam
Autoclave used to sterilize using pressurized steam Heated water ïƒ steam ïƒ increased pressure Preferred method of sterilization Achieves sterilization at 121°C and 15psi in 15 minutes Effective against endospores Flash autoclaving sterilizes at 135°C and 15psi in 3 minutes Prions destroyed at 132°C and 15psi for 4.5 hours
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Heat as Control Dry heat Not as effective as moist heat
Sterilization requires longer times and higher temperatures 200°C for 1.5 hours vs. 121°C for 15 minutes for moist Incineration method of dry heat sterilization Oxidizes cell to ashes Used to destroy medical waste and animal carcasses Flaming laboratory inoculation loop incinerates organism Results in sterile loop
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Other Physical Methods of Control
Heat sensitive materials require other methods of microbial control Filtration Irradiation High-pressure treatment
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Other Physical Methods of Control
Filtration Membrane filtration used to remove microbes from fluids and air Liquid filtration Used for heat sensitive fluids Membrane filters allow liquids to flow through Traps microbes on filter Depth filters trap microbes using electrical charge Filtration of air High efficiency particulate air (HEPA) filter remove nearly all microbes from air Filter has 0.3µm pores to trap organisms
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Other Physical Methods of Control
Radiation Electromagnetic radiation Energy released from waves Based on wavelength and frequency Shorter wavelength, higher frequency = more energy Range of wavelength is electromagnetic spectrum Radiation can be ionizing or non-ionizing
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Other Physical Methods of Control
Ionizing radiation Radiation able to strip electrons from atoms Three sources Gamma radiation X-rays Electron accelerators Causes damage to DNA and potentially to plasma membrane Used to sterilize heat resistant materials Medical equipment, surgical supplies, medications Some endospores can be resistant
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Other Physical Methods of Control
Ultraviolet radiation Non-ionizing radiation Only type to destroy microbes directly Damages DNA Causes thymine dimers Used to destroy microbes in air, drinking water and surfaces Limitation Poor penetrating power Thin films or coverings can limit effect
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Other Physical Methods of Control
High pressure processing Used in pasteurization of commercial foods Does not use high temperatures Employs high pressure Up to 130,000 psi Destroys microbes by denaturing proteins and altering cell membrane permeability
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Chemicals as Control Chemicals can be used to disinfect and sterilize
Called germicidal chemicals Reacts with vital cell sites Proteins DNA Cell membrane
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Chemicals as Control Potency of chemicals Formulations generally contain more than one antimicrobial agent Regulated by FDA Antiseptics EPA Disinfectants Germicidal agents grouped according to potency Sterilants = Destroy all microorganisms High-level disinfectants Destroys viruses and vegetative cells, Not endospores Intermediate-level disinfectants Kills vegetative cells fungi, most viruses, Low-level disinfectants Removes fungi, vegetative bacteria and enveloped viruses Not mycobacteria, naked viruses or endospores
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Chemicals as Control Selecting appropriate chemical Points to consider
Toxicity Benefits must be weighed against risk of use Activity in presence of organic material Many germicides inactivated in presence of organic matter Compatibility with material being treated Liquids cannot be used on electrical equipment Residue Residues can be toxic or corrosive Cost and availability Storage and stability Concentrated stock relieves some storage issues Environmental risk Is germicidal agent harmful to environment
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Chemicals as Control Classes of chemicals
Germicides represent a number or chemical families Alcohols Aldehydes Biguanides Ethylene oxide Halogens Metals Ozone Peroxides Phenolics Quaternary ammonium compounds
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Chemicals as Control Alcohols
Solutions of 60% - 80% isopropyl or ethyl alcohol kill vegetative bacteria and fungi Not effective against endospores and some naked viruses Mode of action Coagulation of proteins and essential enzymes Damage to lipid membranes Commonly used as antiseptic and disinfectant Limitations Evaporates quickly limiting contact time May damage material such as rubber and some plastics
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Chemicals as Control Aldehydes
Destroy organisms by inactivating proteins and DNA 2% glutaraldehyde solution most widely used liquid sterilant Orthophthalaldehyde studied as alternative Formalin used to kill bacteria and inactivate viruses Also used for specimen preservation Formalin is solution made from formaldehyde
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Chemicals as Control Biguanides
Most effective member of group is chlorhexidine Extensively used in antiseptics Relative low toxicity Destroys wide range of organisms
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Chemicals as Control Ethylene oxide Useful gaseous sterilant
Destroys microbes including endospores and viruses Mode of action Reacts with proteins Useful in sterilizing heat or moisture sensitive items Limitations Mutagenic and potentially carcinogenic
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Chemicals as Control Halogens Common disinfectants Mode of action
Oxidizing proteins and other cell components Includes chlorine and iodine Chlorine Destroys all types of organisms and viruses Used as disinfectant Caustic to skin and mucous membranes Chlorine dioxide replacing chlorine in many applications Iodine Kills vegetative cells Not reliable with endospores Used in tincture or iodophore on skin
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Chemicals as Control Metal compounds
Compounds combine with enzymes and proteins Interfering with function High concentrations of many metals toxic to human tissue Silver still used as disinfectant Creams containing silver sulfadiazine used to prevent secondary infections Also available on bandages for wound care
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Chemicals as Control Ozone O3 Powerful oxidizing agent
Unstable form of oxygen Powerful oxidizing agent Used as alternative to chlorine As disinfectant for drinking and waste water
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Chemicals as Control Peroxygens
Includes hydrogen peroxide and peracetic acid Powerful oxidizing agents Readily biodegradable Less toxic than ethylene oxide and glutaraldehyde
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Chemicals as Control Hydrogen peroxide Peracetic acid
Effectiveness depends on surface being treated Living tissue produce catalase enzyme Breaks down hydrogen peroxide to oxygen and water More effective on inanimate object Useful as disinfectant Leaves no residue Doesn’t damage most materials Hot solutions used in food industry Vapor-phase can be used as sterilant Peracetic acid More potent then hydrogen peroxide Effective on organic material Can be used on wide range of material
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Chemicals as Control Phenolics A.k.a carbolic acid
One of the earliest disinfectants Now has limited use Active ingredient in Lysol Mode of action Destroy plasma membrane Denature proteins Kills most vegetative cells Can kill mycobacterium at high concentrations Not reliable on all groups of viruses Triclosan and hexachlorophene phenols used in soaps and lotions
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Chemicals as Control Quaternary ammonium compounds A.k.a Quats
Cationic detergents Nontoxic Used to disinfect food preparation surfaces Mode of action Reduces surface tension Aids in removal of dirt and organic matter Facilitates mechanical removal of organisms Positive charge attracts Quats to negative charge of cell surface Reacts with membrane Destroys vegetative bacteria and enveloped viruses Not effective on endospores, mycobacteria and naked viruses
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Preservation of Perishable Products
Preservation extends shelf-life of many products Chemicals are often added to prevent or slow growth of microbes Other methods include Low temperature storage Freezing Reducing available water
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Chemicals as Control Chemical preservatives
Numerous chemicals are used as preservatives Formaldehyde, Quats, and phenols Weak organic acids often used as food preservatives Benzoic, ascorbic and propionic acids Used in bread, cheese and juice Mode of action Alter cell membrane function Interfere with energy transformation Nitrates and nitrites used in processed meats Inhibits germination of endospores and growth of vegetative cells Have been shown to be potent carcinogen
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Chemicals as Control Low temperature storage
Microbial growth is temperature dependent Low temperatures slow down or stop enzymatic reactions of mesophiles and thermophiles Some psychrophiles still able to grow Freezing means of food preservation Essentially stops microbial growth Irreversibly damages cell Kills up to 50% of microbes Remaing cells still pose potential threat
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Chemicals as Control Reducing water availability
Decreasing water availability accomplished by salting or drying food Addition of salt increases environmental solutes Causes cellular plasmolysis Numerous bacteria can continue to grow in high salt environments Staphylococcus aureus can survive in high salt concentrations Desiccation or drying is often supplemented by other methods Salting Lyophilization (freeze drying) Widely used to preserve foods like coffee, milk and meats
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