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Control of microbial growth Some terms to start with. terms • • • • Biocide: kills biologicals Germicide: Kills bacteria Bacteriostasis: stops growth Asepsis: absence of growth Some terms. • Sterilization: destruction of all life • Disinfecting: destruction of vegetative pathogens • Antisepsis: disinfection of living tissue • Sepsis: refers to microbial contamination. • Aseptic surgery techniques prevent microbial contamination of wounds. Factors that influence effectiveness • • • • Number of microbes Environmental influences Time exposure Microbial characteristics • Bacterial populations die at a constant logarithmic rate. Figure 7.1a Methods of Control • • • • Physical Chemical Biological Why is prevention best? Physical Controls: • Not for use on living organisms • Somehow, alter membrane permeability and / or structure of proteins and nucleic acids The autoclave: Moist heat and pressure • 15psi, 121’c, 15 minutes • Thermal death point (TDP): Lowest temperature at which all cells in a culture are killed in 10 min. • Thermal death time (TDT): Time to kill all cells in a culture • Decimal reduction time (DRT): Minutes to kill 90% of a population at a given temperature Other controls • • • • • Heat, dry heat takes longer Low temperature Desiccation Osmotic Pressure Radiation – Ionizing – No ionizing (UV) •Radiation damages DNA •Ionizing radiation (X rays, gamma rays, electron beams) •Nonionizing radiation (UV) •(Microwaves kill by heat; not especially antimicrobial) • Filtration Removes Microbes Chemical Methods of Control • May be used on living organisms • Factors that may effect disinfecting – Concentration of the disinfectant – What is to be disinfected • pH • Organic materials – Contact with microbe – time Testing • Use dilution test – Three microbes are used Salmonella choleraesuis Staphylococcus aureus Pseudomonas aeruginosa – Metal carriers dipped and dried – Standard conditions 10 min at 20’C Disk Diffusion Method Types of disinfectants • Phenol and Phenolics • Halogens (iodine, chlorine) bind to aa of enzymes – Betadine iodophor (iodine bound to organic molecules) Types of Disinfectants • Phenol • Phenolics. Lysol • Bisphenols. Hexachlorophene, Triclosan – Disrupt plasma membranes Figure 7.7 • Alcohol’s usually by protein denaturizing • Heavy metals – Oligodynamic action – Denature proteins • Surface active Agents (detergents) Soap Degerming Acid-anionic detergents Sanitizing Quarternary ammonium compounds Cationic detergents Bactericidal, Denature proteins, disrupt plasma membrane • Antibiotics – Worry about resistance – Not many used as preservative (Nisin, Natamycin) • Gaseous Chemostaerilizers – Ethylene oxide denatures proteins • Resistance to these methods depends on the microbe. Types of Disinfectants • Peroxygens – Oxidizing agents – O3, H2O2, peracetic acid Types of Disinfectants • Aldehydes – Inactivate proteins by cross-linking with functional groups (–NH2, –OH, –COOH, — SH) – Glutaraldehyde, formaldehyde Types of Disinfectants • Chemical Food Preservatives – Organic Acids • Inhibit metabolism • Sorbic acid, benzoic acid, calcium propionate • Control molds and bacteria in foods and cosmetics – Nitrite prevents endospore germination – Antibiotics. Nisin and natamycin prevent spoilage of cheese Resistance to chemical Biocides