Download Using Chemicals to Destroy Microorganisms and Viruses

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
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Heat
Irradiation
Filtration
Mechanical removal
– Chemical methods
• Use a variety of antimicrobial chemicals
• Chemical depends on circumstances and degree of control
required
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 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 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 Control
• Situational
considerations
– Microbial control
methods are highly
variable
• Depends on situation and
degree of control required
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Daily life
Hospital
Microbiology laboratories
Food and food
production facilities
– Water treatment
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 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 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 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 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 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
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
Protozoans
Giardia lamblia and Cryptosporidium parvum
Cause digestive problems
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
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
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
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 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 Control
• Moist heat
– Destroys through irreversible coagulation of
proteins
– Moist heat includes
• Boiling
• Pasteurization
• Pressurized steam
Heat as Control
• Boiling (100° C)
– Destroys most microorganisms and viruses
– Not effective means of sterilization
• Does not destroy endospores
• Pasteurization
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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 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
• Dry heat
Heat as Control
– 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
• Heat sensitive materials require other
methods of microbial control
– Filtration
– Irradiation
– High-pressure treatment
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 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 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 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 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 can be
used to disinfect and
sterilize
– Called germicidal
chemicals
• Reacts with vital cell
sites
– Proteins
– DNA
– Cell membrane
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
– 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
• 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 Control
• Classes of chemicals
– Germicides represent a number or chemical families
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Alcohols
Aldehydes
Biguanides
Ethylene oxide
Halogens
Metals
Ozone
Peroxides
Phenolics
Quaternary ammonium compounds
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 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 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 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