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Using Chemicals to Destroy Microorganisms and Viruses. Chapter 5. 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

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approaches to control
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
approaches to control1
Approaches to Control
  • Principles of control
    • Sterilization
      • 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
approaches to control2
Approaches to Control
  • Principles of control
    • Pasteurization
      • Brief heat treatment used to reduce organisms that cause food spoilage
        • Surfaces can also be pasteurized
    • Decontamination
      • Treatment to reduce pathogens to level considered safe to handle
    • Degerming
      • Mechanism uses to decrease number of microbes in an area
        • Particularly the skin (antiseptics)
approaches to control3
Approaches to Control
  • Principles of control
    • 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
approaches to control4
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
approaches to control5
Approaches to Control
  • Daily life
    • Washing and 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 organisms from outer layer of skin
          • Normal flora usually unaffected because it resides in deeper layers
approaches to control6
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
approaches to control7
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
          • Eliminates contamination of environment
approaches to control8
Approaches to Control
  • Food and food production facilities
    • Retention of 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
approaches to control9
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 organisms resistant to chemical disinfectants
selection of antimicrobial procedure
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
selection of antimicrobial procedure1
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

Bacterial endospores

Clostridium botulinum – causes botulism, resists boiling, but autoclaving kills


Giardia lamblia and Cryptosporidium parvum

Cause digestive problems

selection of antimicrobial procedure2
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
selection of antimicrobial procedure3
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
selection of antimicrobial procedure4
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
selection of antimicrobial procedure5
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
heat as control
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
heat as control1
Heat as Control
  • Moist heat
    • Destroys through irreversible coagulation of proteins
    • Moist heat includes
      • Boiling
      • Pasteurization
      • Pressurized steam
heat as control2
Heat as Control
  • Boiling (100° C)
    • 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
heat as control3
Heat as Control
  • Pressurized steam
    • Autoclave used to sterilize using pressurized steam
      • Heated water  steam  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
heat as control4
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
    • 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
other physical methods of control
Other Physical Methods of Control
  • Heat sensitive materials require other methods of microbial control
    • Filtration
    • Irradiation
    • High-pressure treatment
other physical methods of control1
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 removes nearly all microbes from air
      • Filter has 0.3µm pores to trap organisms
other physical methods of control2
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
other physical methods of control3
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
other physical methods of control4
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
other physical methods of control5
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
chemicals as control
Chemicals as Control
  • Chemicals can be used to disinfect and sterilize
    • Called germicidal chemicals
  • Reacts with vital cell sites
    • Proteins
    • DNA
    • Cell membrane
chemicals as control1
Potency of chemicals

Formulations generally contain more than one antimicrobial agent

Regulated by





Germicidal agents grouped according to potency

Sterilants =

Destroy all microorganisms

High-level disinfectants

Destroy viruses and vegetative cells,

Not endospores

Intermediate-level disinfectants

Kill vegetative cells fungi, most viruses,

Not endospores

Low-level disinfectants

Removes fungi, vegetative bacteria and enveloped viruses,

Not mycobacteria, naked viruses or endospores

Chemicals as Control
chemicals as control2
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

Chemicals as Control

  • Selecting appropriate chemical
    • Points to consider
      • 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
chemicals as control3
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
preservation of perishable products
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
chemicals as control4
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
chemicals as control5
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 as means of food preservation
      • Essentially stops microbial growth
      • Irreversibly damages cell
        • Kills up to 50% of microbes
          • Remaining cells still pose potential threat
chemicals as control6
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