Definitions. BIOL 3702: Chapter 8. Control of Microbes in the Environment. Mechanical Removal Methods. Pattern of Microbial Death

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1 Definitions Control of Microbes in the Environment u Sterilization - destruction or removal of all viable organisms from an object or environment (agent = sterilant) u Disinfection - killing, inhibition, or removal of pathogenic microorganisms (mainly pertains to inanimate objects) (agent = disinfectant) u Sanitization - reducing microbial populations to a safe level in accord with public health standards (agent = sanitizer) Slide No. 1 Slide No. 2 Definitions (cont.) Definitions (cont.) u Antisepsis - prevention of microbial infection on living tissue (agent = antiseptic) u Chemotherapy - prevention of microbial infection within living tissue Figure 8.2 u -cide - a suffix indicating that the agent will kill the kind of organism in question (e.g., viricide) u -static - a suffix indicating that the agent will prevent the growth of the type of organism in question (e.g., bacteriostatic) Slide No. 3 Slide No. 4 Pattern of Microbial Death u Microorganisms are not killed instantly when exposed to a lethal agent u Population death decreases by a constant fraction at constant intervals (exponential killing) u A microorganism is considered dead when it is unable to grow in conditions that would normally support its growth Mechanical Removal Methods u Filtration - sterilizes heat-sensitive liquids and gases by removing microorganisms rather than destroying them v Depth filters - thick fibrous or granular filters that remove microbes by physical screening, entrapment, or adsorption v Membrane filters - thin filters with defined pore sizes that remove microorganisms v High-efficiency particulate air (HEPA) filters - used in biological safety cabinets to sterilize air Slide No. 5 Slide No. 6 Dr. Cooper 1

2 Spring 2015 Filter unit and filtration apparatus Figure 8.4a Slide No. 7 Figure 8.5 Slide No. 9 Slide No. 8 Figure 8.6 Slide No. 10 Slide No. 12 Physical Methods of Control u Heat v Moist heat Ø Boiling water is effective against vegetative cells and eukaryotic spores, but not bacterial spores Ø Autoclaving (steam under pressure) is effective against vegetative cells and most bacterial endospores Ø Quality control - includes strips with Geobacillus stearothermophilus Dr. Cooper Slide No. 11 2

3 Spring 2015 The Autoclave Figure 8.7 Slide No. 13 v Pasteurization Ø A process involving brief exposure to temperatures below the boiling point of water Ø Reduces the total microbial population Ø NOT STERILIZATION Ø Often used for heat-sensitive materials v Tyndallization Ø A process involving multiple brief exposures to temperatures below the boiling point of water Ø Used for materials that cannot withstand the high temperatures of autoclaving Slide No. 14 v Dry heat can be used to sterilize moisture- sensitive materials such as powders, oils, and similar items Ø Less efficient than moist heat Ø Usually requires higher temperatures (160 to 170 C) and longer exposure times (2 to 3 hrs) v Incineration often used to sterilize re-usable items, e.g., microbiological loops Figure 8.8 Slide No. 15 Slide No. 16 u Radiation v Ultraviolet (UV) radiation is effective, but its use is limited to surface sterilization because UV radiation does not penetrate glass, dirt films, water, and other substances v Ionizing radiation (X rays, gamma rays, etc.) is effective and penetrates the material Figure 8.10 Dr. Cooper Slide No. 17 Slide No. 18 3

4 Chemical Agents in Control u Disinfectant must be effective against wide variety of infectious agents at low concentrations v Must be effective in the presence of organic matter; should be stable in storage v Overuse of antiseptics such as triclosan has selected for triclosan resistant bacteria and possibly antibiotic resistant u Phenolics v Laboratory and hospital disinfectants v Act by denaturing proteins and cell membranes u Alcohol v Widely used disinfectants and antiseptics v Will not kill endospores v Act by denaturing proteins and possibly by dissolving membrane lipids Slide No. 19 Slide No. 20 u Halogens v Widely used antiseptics and disinfectant v Examples Ø Iodine: oxidizes cell constituents and iodinates cell proteins Ø Chlorine: oxidizes cell constituents u Heavy metals v Effective but usually toxic v Act by combining with proteins and inactivating them u Aldehydes v Reactive molecules that can be used as chemical sterilants, but may irritate the skin v Act by combining with proteins and inactivating them Figure 8.11 Slide No. 21 Slide No. 22 u Quaternary ammonium compounds v Cationic detergents of low toxicity v Uses: Ø Disinfectants for food utensils and small instruments Ø Skin antiseptic v Act by disrupting biological membranes and possibly by denaturing proteins u Sterilizing gases (e.g., ethylene oxide) v Used to sterilize heat-sensitive materials v Act by combining with proteins and inactivating them u Vapor-phase hydrogen peroxide has been used to decontaminate biological safety cabinets Slide No. 23 Slide No. 24 Dr. Cooper 4

5 Conditions Affecting Antimicrobial Activity u Population size - larger populations take longer to kill than smaller populations u Population composition - microorganisms differ markedly in their sensitivity to various agents u Concentration or intensity of the antimicrobial agent - higher concentrations or intensities are generally more efficient, but the relationship is not linear Conditions Affecting Antimicrobial Activity (cont.) u Duration of exposure - longer the exposure, the greater the number of organisms killed u Temperature - higher temperatures will often (but not always) increase the effectiveness of killing u Local environment - environmental factors, such as ph, viscosity, and concentration of organic matter, can profoundly influence the effectiveness of a antimicrobial agent Slide No. 25 Slide No. 26 Biological Control of Microbes u Emerging field showing great promise u Natural control mechanisms v Predation by Bdellovibrio v Viral-mediated lysis using pathogen specific bacteriophage lysins v Toxin-mediated killing using bacteriocins Annual Review of Microbiology 63: , 2009 Slide No. 27 Dr. Cooper 5