methods for control of microbial growth. controlling microorganisms physical, chemical, and...
TRANSCRIPT
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Methods for Control of Microbial Growth
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Controlling Microorganisms
• Physical, chemical, and mechanical methods can be used to destroy or reduce undesirable microbes in a given area
• Primary targets are microorganisms capable of causing infection or spoilage:– vegetative bacterial cells and endospores– fungal hyphae and spores, yeasts– protozoan trophozoites and cysts– worms– viruses
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Hierarchy of Resistance
• Most resistant = bacterial endospores
• Moderately resistant = cysts, fungal zygospores, naked viruses
• Least resistant = vegetative bacterial cells
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Types of Control
• Sterilization – a process that destroys all viable microbes, including viruses and endospores; microbiocidal
• Disinfection – a process to destroy vegetative pathogens, not endospores; for inanimate objects
• Antisepsis – disinfectants applied directly to exposed body surfaces
• Sanitization – any cleansing technique that mechanically removes microbes
• Degermation – reduces the number of microbes
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A Number of Factors Influence the Effectiveness of Control Agents
• Number of microbes
• Nature of microbes in the population
• Temperature and pH of environment
• Concentration or dosage of agent
• Mode of action of the agent
• Presence of solvents, organic matter, or inhibitors
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Microbiocidals Cause Microbial Death
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Stasis Agents Slow or Retard Growth, but Do Not Kill Microbes
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Microbial Death
The Permanent Loss of Reproductive Capability Under
Optimal Growth Conditions
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Cellular Targets of Control
1. Cell wall
2. Cell membrane
3. Nucleic Acids (DNA, RNA)
4. Proteins
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Physical Control Practices
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Heat-Based Microbial Control Processes
• Incineration/ baking achieve sterility• Autoclaving: sterilization with live
steam and pressure, very widely used• Pasteurization: flash heat treatment
(63°C - 66°C for 30 minutes) that reduces the bio-burden of food materials (kills Salmonella and Listeria)
• Boiling disinfection – does not achieve sterility
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Radiation Sterilization
• Ionizing radiation (X-rays, gamma particles) penetrates and damages DNA and other vital cell components
• Non-ionizing radiation (UV light) causes adjacent T-T pairs in DNA to fuse
• UV has limited penetration; use for surfaces and films of liquids
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Ultra-filtration
• Mechanically excludes organisms from a liquid
• Membranes have a specific pore size; any particle larger cannot pass through
• Achieves sterilization
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Filtration
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Targets of Chemical Agents
• Cell Membrane - detergents
• Key Proteins – denaturing and cross-linking agents
• Nucleic Acids – alkylating and cross-linking agents
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Halogen Antimicrobials - Denature Proteins
• Chlorine compounds (gaseous Cl, bleach, chloramine); can be sporicidal
• Iodine (tincture, Betadine)
• Halogens can react with any organic matter – a surface should be clean before applying them!
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Phenolics- Disrupt Cell Membranes & Precipitate Proteins
• Phenol
• Lysol
• PhisoHex (not any more)
• Benzalkonium chloride
• Triclosan
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Alcohols: Dissolve Membranes and Coagulate Proteins
• Ethanol (70%)
• Isopropanol
• Act as surfactants dissolving membrane lipids and coagulating proteins of vegetative bacterial cells and fungi
• Cannot destroy spores at room temperature
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Hydrogen Peroxide – Attacks DNA and Proteins
3% solution is effective as wound antiseptic, but is
potentially damaging to tissues
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Heavy Metals
Mercury, Silver Salts Kill Vegetative Cells Present in low Numbers by
Inactivating Proteins
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Aldehydes – Cross-link DNA and Proteins
• Formaldehyde – formalin
• Glutaraldehyde - Cidex
• A soak of dental or surgical instruments in glutaraldehyde Does Not guarantee sterility!
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Gases and Aerosols
• Ethylene oxide, propylene oxide
• Strong alkylating agents
• High level
• Sterilize and disinfect plastics and prepackaged devices, foods