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Heat denatures enzymes•Thermal death point (TDP):lowest temperature at which all cells in a liquid culture are killed in 10 min•Thermal death time (TDT):minimal time for all bacteria in a liquid culture to be killed at a particular temperature•Decimal reduction time (DRT)•Minutes to kill 90% of a specific population of bacteria at a given temperature•Pasteurizationreduces spoilage organisms and pathogens•High-temperature short-time (HTST):72°C for 15 sec•Thermoduricorganisms survive
Moist Heat Sterilization•Moist heat coagulates/denatures proteins•Boiling•Free-flowing steam•Autoclave:steam under pressure•121°-C at 15 psi for 15 min•Kills all organisms and endospores•Steam must contact the item's surface•Large containers require longer sterilization times•Test strips are used to indicate sterilityDry Heat Sterilization•Kills by oxidation–Flaming–Incineration–Hot-air sterilizationFiltration•Passage of substance through a screenlike material•Used for heat-sensitive materials•High-efficiency particulate air (HEPA) filtersremove microbes > 0.3 μm in diameter•Membrane filtersremove microbes > 0.22 μm–Pore sizes of as small as 0.01 μm are available, whichcan filter out viruses and large proteinsRadiation•Ionizing radiation(X-rays, gamma rays, electron beams)–Ionizes water to create reactive hydroxyl radicals–Damages DNA by causing lethal mutations•Nonionizing radiation(ultraviolet, 260 nm)–Damages DNA by creating thymine dimers•Microwaveskill by heat; not especially antimicrobialPhysical Methods•Low temperature has a bacteriostatic effect•Refrigeration•Deep-freezing•Lyophilization (freeze drying)•High pressure denatures proteins•Desiccation: absence of water prevents metabolism•Osmotic pressure uses high concentrations of salts and sugars to create hypertonic environment; causes plasmolysis
Chemical•Phenol and Phenolics: Injure lipids of plasma membranes, causing leakage•Bisphenols: Contain two phenol groups connected by a bridge. Hexachlorophene and triclosan. Disrupt plasma membranes•Biguanides: Chlorhexidine. Used in surgical hand scrubs. Disrupt plasma membranesEssential Oils•Mixtures of hydrocarbons extracted from plants•Peppermint oil, pine oil, orange oil•Used for centuries in traditional medicine and for preserving food•Microbial action primarily due to phenolics and terpenes•Stronger activity against gram-positive bacteria; effectiveness against viruses not studied to dateHalogens•Iodine•Tincture: solution in aqueous alcohol•Iodophor: combined with organic molecules•Impairs protein synthesis and alters membranes•Chlorine•Oxidizing agents; shut down cellular enzyme systems•Bleach: hypochlorous acid (HOCl)•Chloramine: chlorine + ammoniaAlcohols•Denatures proteins and dissolves lipids•No effect on endospores and nonenveloped viruses•Ethanol and isopropanol•Require waterHeavy Metals and their Compounds•Oligodynamic action—very small amounts exert antimicrobial activity•Denature proteins••