Download CH05-Control of Microbial Growth

5.5. Using Chemicals to Destroy Microorganisms and
Viruses Operating Room: Chemical Treatment of Soldier
 Germicidal Chemicals
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Used to disinfect and sterilize
Used to treat large surfaces and heat sensitive items
Chemicals react with cell components
Less reliable than heat
Toxic or Non-toxic
 Potency of Germicidal Chemical Formulations
1)
Sterilants
• Destroy all microorganisms…including endospores
• Heat-sensitive critical instruments
2)
High-level disinfectants
• Destroy viruses, vegetative cells
• Do not reliably kill endospores
• Semi-critical instruments
3)
Intermediate-level disinfectants
• Destroy vegetative bacteria, mycobacteria, fungi, and most viruses
• Disinfect non-critical instruments
4)
Low-level disinfectants
• Destroy fungi, vegetative bacteria except mycobacteria, and enveloped viruses
• Do not kill endospores, naked viruses
• Disinfect furniture, floors, walls
5.5. Using Chemicals to Destroy Microorganisms and
Viruses
 Selecting the Appropriate Germicidal
Chemical
1) Toxicity
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benefits must be weighed against risk of use
2) Activity in presence of organic matter
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Many germicides inactivated
3) Compatibility with material being treated
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Liquids cannot be used on electrical
equipment…use gases
4) Residues:
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can be toxic or corrosive
5) Cost and availability
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Less $ = readily available
6) Storage and stability
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Concentrated stock decreases storage space
7) Environmental risk
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Agent may need to be neutralized before
disposal
Classes of Germicidal Chemicals
Classes of Germicidal Chemicals
 Alcohols
• 60–80% aqueous solutions of ethyl or isopropyl alcohol
• Kills vegetative bacteria and fungi
• Not reliable against endospores, some naked viruses
• Commonly used as antiseptic and disinfectant
• Limitations
• Evaporates quickly, limiting contact time
• Can damage rubber, some plastics, and others
 Aldehydes
• Glutaraldehyde, formaldehyde, and orthophthalaldehyde (OPA)
• Inactivates proteins and nucleic acids
• 2% alkaline glutaraldehyde common sterilant
• Immersion for 10–12 hours kills all microbial life
• Formaldehyde
• Effective germicide that kills most microbes quickly
• Used to kill bacteria and inactivate viruses for vaccines
• Used to preserve specimens
Classes of Germicidal
Chemicals
 Biguanides
• Chlorhexidine most effective
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Extensive in antiseptics
Stays on skin, mucous membranes
Relatively low toxicity
Destroys vegetative bacteria, fungi, some
enveloped viruses
• Common in many products: skin cream,
mouthwash
 Ethylene oxide
• Gaseous sterilant
• Destroys microbes including endospores and
viruses
• Reacts with proteins
• Penetrates fabrics, equipment, implantable devices
• Pacemakers, artificial hips
• Useful in sterilizing heat- or moisture-sensitive items
• Applied in special chamber resembling autoclave
• Limitations: mutagenic and potentially carcinogenic
Classes of Germicidal Chemicals
 Halogens
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• oxidize proteins, cellular components
Chlorine: Destroys all microorganisms and viruses
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Used as disinfectant…E.g., swimming pools, water, waste liquids
Caustic to skin and mucous membranes
1:100 dilution of household bleach effective
Very low levels disinfect drinking water…E.g., 2 drops/liter @ 30 min
• Cryptosporidium oocysts, Giardia cysts survive
Iodine: Kills vegetative cells, unreliable on endospores
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Commonly used as iodophore
• Iodine slowly released from carrier molecule
 Metal Compounds
• Combine with sulfhydryl groups of
enzymes, proteins
• High concentrations too toxic to be used
medically
• Silver still used as disinfectant: creams,
bandages
• Silver nitrate eyedrops were required to
prevent Neisseria gonorrhoeae
infections acquired during birth
Argyria – colloidal silver
Classes of Germicidal Chemicals
 Ozone
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O3: unstable form of oxygen
Decomposes quickly, so generated on-site
Powerful oxidizing agent
Used as alternative to chlorine
• Disinfectant for drinking and wastewater
 Peroxygens
• Powerful oxidizers used as sterilants
• Readily biodegradable, no residue
• Less toxic than ethylene oxide,
glutaraldehyde
• Hydrogen peroxide: effectiveness depends
on surface….inanimate or living
• Aerobic cells produce enzyme catalase
– Breaks down H2O2 to O2, H2O
• More effective on inanimate object
• Peracetic acid: more potent than H2O2
• Effective on organic material
• Useful on wide range of material
Classes of Germicidal Chemicals
 Phenolic Compounds (Phenolics)
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Phenol one of earliest disinfectants
Has unpleasant odor, irritates skin
Phenolics kill most vegetative bacteria
• Mycobacterium at high concentrations
• Not reliable on all virus groups
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Destroy cytoplasmic membranes,
denature proteins
Wide activity range, reasonable cost,
remain effective in presence of detergents
and organic contaminants
• Leave antimicrobial residue
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Some sufficiently non-toxic; used in
soaps, lotions
• Triclosan, hexachlorophene
 Quaternary Ammonium Compounds (Quats)
• Cationic (positively charged) detergents
• Nontoxic, used to disinfect food
preparation surfaces
• Charged hydrophilic and uncharged
hydrophobic regions
• Reduces surface tension of liquids
• Aids in removal of dirt, organic matter,
organisms
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Most household soaps, detergents are
anionic
But positive charge of quats attracts them
to negative charge of cell surface
• Reacts with membrane
• Destroys vegetative bacteria and
enveloped viruses
• Not effective on endospores,
mycobacteria, naked viruses
5.6. Preservation of Perishable Products
 Chemical preservatives
• Food preservatives must be non-toxic for safe ingestion
• Weak organic acids (benzoic, sorbic, propionic)
• Inhibit metabolism, alter cell membrane function
• Control molds and bacteria in foods and cosmetics
• Nitrate and nitrite used in processed meats
• Inhibit endospore germination and vegetative cell growth
– Stops growth of Clostridium botulinum
– Higher concentrations give meats pink color
• Shown to be carcinogenic—form nitrosamines
5.6. Preservation of Perishable Products
 Low-Temperature Storage
• Refrigeration inhibits growth of pathogens and spoilage organisms by
slowing or stopping enzyme reactions
• Psychrotrophs, psychrophilic organisms can still grow
• Freezing preserves by stopping all microbial growth
• Some microbial cells killed by ice crystal formation, but many survive and
can grow once thawed
5.6. Preservation of Perishable Products
 Reducing Available Water
• Accomplished by salting, adding sugar, or drying food
• Addition of salt, sugar increases environmental solutes
• Causes cellular plasmolysis (water exits bacterial cells)
• Some bacteria grow in high salt environments
• Staphylococcus aureus
• Drying often supplemented by salting
• Lyophilization (freeze drying) foods
• Coffee, milk, meats, fruits, vegetables
• Drying stops microbial growth but does not reliably
kill
• Numerous cases of salmonellosis from dried eggs
Chapter 05
Control of Microbial Growth
 Covers methods to:
 Destroy
 Remove
 Inhibit
 Microbial growth on:
 Inanimate objects
 Some body
surfaces
 Methods can damage
all forms of life
 Antibiotics and
antimicrobial
medications target
only microbes…CH20
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