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Control of Microbial Growth (page 1)(This chapter has 6 pages)© Kenneth Todar, PhDIntroduction
In the 19th century, surgery was risky anddangerous, and patients undergoing even the most routine operationswere at very high risk of infection. This wasso because surgery was not performed under asepticconditions. The operating room, the surgeon"s hands, and thesurgical instruments were laden with microbes, which caused high levelsof infection and mortality. Surgeons in the mid-1800s often operated wearing their street clothes,without washing their hands. They frequently used ordinary sewingthread to suture wounds, and stuck the needles in the lapels of theirfrock coats in between patients. Surgical dressings were oftenmade up of surplus cotton or jute from the floors of cotton mills. Itwas against this background that French scientist Louis Pasteurdemonstrated that invisible microbes caused disease.
In microbiology, sterilization refers tothe complete destructionor elimination of all viable organisms in or on a substance beingsterilized.There are no degrees of sterilization: an object or substance is eithersterile or not.Sterilization procedures involve the use of heat, radiation orchemicals,or physical removal of cells.
Methods of SterilizationHeat: most importantand widely used. For sterilization one must consider the type ofheat, and most importantly, the timeofapplication and temperatureto ensure destruction of all microorganisms.Endospores of bacteria are considered the most thermoduric of all cellsso their destruction guarantees sterility.
Incineration: burns organisms andphysicallydestroys them. Used for needles, inoculating wires, glassware, etc.andobjects not destroyed in the incineration process.
Boiling: 100o for 30 minutes.Kills everything except some endospores. To kill endospores, andtherefore sterilize a solution,very long (>6 hours) boiling, or intermittent boiling isrequired (See Table 1 below).
Autoclaving (steam under pressure or pressurecooker) Autoclaving is the most effective and mostefficient means of sterilization. All autoclaves operate on atime/temperature relationship. These two variables are extremelyimportant. Higher temperatures ensure more rapid killing. The usualstandardtemperature/pressure employed is 121ºC/15 psi for 15 minutes.Longer times are needed for larger loads, largevolumes of liquid, and more dense materials. Autoclaving is ideal forsterilizing biohazardous waste, surgical dressings, glassware, manytypes of microbiologic media, liquids, and many other things. However,certain items, such as plastics and certain medical instruments (e.g.fiber-optic endoscopes), cannot withstandautoclaving and should be sterilized with chemical or gas sterilants.When proper conditions and time are employed, no living organisms willsurvive a trip through an autoclave.
Schematic diagram of a laboratoryautoclave in useto sterilize microbiological culture medium. Sterilizationof microbiological culture media is is often carried out with theautoclave. Whenmicrobiological media are prepared, they must be sterilized andrendered free of microbial contamination from air, glassware, hands,etc. The sterilization process is a 100% kill, and guaranteesthat themedium will stay sterile unless exposed to contaminants.
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Why is an autoclave such an effectivesterilizer? The autoclave is a large pressure cooker; it operates byusing steam under pressure as the sterilizing agent. High pressuresenable steam to reach high temperatures, thus increasing its heatcontent and killing power. Most of the heating power of steam comesfrom its latent heat of vaporization. This is the amount of heatrequired to convert boiling water to steam. This amount of heat islarge compared to that required to make water hot. For example, ittakes 80 calories to make 1 liter of water boil, but 540 calories toconvert that boiling water to steam. Therefore, steam at 100º Chas almost seven times more heat than boiling water.
Moist heat is thought to kill microorganisms bycausing denaturation of essential proteins. Death rate is directlyproportional to the concentration of microorganisms at any given time.The time required to kill a known population of microorganisms in aspecific suspension at a particular temperature is referred to as thermal death time (TDT). Increasingthe temperature decreases TDT, and lowering the temperature increasesTDT. Processes conducted at high temperatures for short periods of timeare preferred over lower temperatures for longer times.Environmental conditions also influence TDT. Increased heat causesincreased toxicity of metabolic products and toxins. TDT decreases withpronounced acidic or basic pHs. However, fats and oils slow heatpenetration and increase TDT. It must be remembered that thermal deathtimes are not precise values; they measure the effectiveness andrapidity of a sterilization process. Autoclaving 121ºC/15 psi for 15 minutes exceeds the thermal death time for most organisms except someextraordinary sporeformers.Dry heat (hot air oven): basically the cooking oven. The rules ofrelating time and temperature apply, but dry heat is not as effectiveasmoist heat (i.e., higher temperatures are needed for longer periods oftime). For example 160o/2hoursor 170o/1hour is necessary for sterilization. The dry heatoven is used forglassware, metal, and objects that won"tmelt.chapter continuedNext Page© Kenneth Todar, Ph.D. All rights reserved. - www.tastecraftedmcd.com