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Microbial control terminology
-Disinfection: reducing or inhibiting microbial growth on a nonliving surface -Antisepsis: reducing or inhibiting microorganisms on living tissue
Rate of Microbial death
bacterial populations subjected to heat or antimicrobial chemicals usually die at a constant rate. Time to kill a population is proportional to the number of microbes. Organic matter may interfere with heat treatments and chemical control agents. longer exposure at low temperatures also kills bacteria.
Actions of Microbial Control Agents
Alteration of Membrane permeability -certain chemical control agents damage lipids / proteins and alter permeability Damage to Proteins and Nucleic Acids -some microbial control agents damage cellular proteins by breaking hydrogen and covalent bonds others interfere with DNA / RNA replication and protein synthesis
Microbial control agents
High temperature Filtration Low Temperature Desiccation Osmotic Pressure Radiation Chemicals
High Temperature
-used to kill or destroy microorganisms -moist heat kills by denaturing enzymes -Thermal Death Point (TDP) is lowest temp at which all bacteria in a liquid culture will be killed in ten minutes -Thermal Death Time (TDT) is length of time required to kill all bacteria in a liquid culture at a given temperature -Decimal reduction time (DRT or D value) is the time it takes at a certain temperature to reduce a microbial population by 90 percent (10 fold, or one log) -Boiling kills many cells and viruses after ten minutes -Autoclaving steam under 15 pounds per square inch pressure
-passage of liquid or gas through a filter with pores small enough to catch microbes (.45 to .2 micrometers) -can be removed from air via high efficiency particulate air filters (hepa filter) -nitrocellulose or cellulose acetate membrane filters can be used to filter bacteria viruses and large proteins
Low Temperature
-effectiveness depends on particular organism and intensity of application -most organisms (except psychrophiles) do not reproduce at ordinary refrigerator temps (0 to 7C) -organisms can be well preserved in liquid nitrogen
In the absence of water microorganisms cannot grow but can remain viable -viruses & endospores resist desiccation -salmonella survives dry conditions
Osmotic Pressure
-microorganisms in high concentrations of salts and sugars undergo plasmolysis -molds and yeasts are more capable than bacteria of growing in environments with low moisture or high osmotic pressure
-effects are dependant on wavelength intensity and duration of radiation exposure -Ionizing radiation (gamma rays, X rays, high energy electron beams) high degree of penetration works mostly by ionizing water and forming highly reactive hydroxyl radicals -UV radiation, non-ionizing, low degree of penetration, causes thiamine dimers in DNA that interfere with replication, most effective germicidal wavelength is 260nm -microwaves (kill indirectly as things get hot)
Chemical methods of Microbial control
-Few chemical agents achieve sterility 50% to 70% ethanol phenolic compounds (lysol) tincture of iodine (2% in 70% alcohol ethylene oxide gas chlorine
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