w2 7.Contrel of rganisms by Physical and Chemical Agent it. aoBiological Safety Radiation opcrates ear Tampa.Florida.However,this process has not yet objects are briefly described next. and the F fostenliz 3 How are n ization flash ization ultrshioh 4.How can low temperature be used to control microorganisms Ionizing radiation is an e trates deep troy bacte afety cabine each is usePrescott−Harley−Klein: Microbiology, Fifth Edition II. Microbial Nutrition, Growth, and Control 7. Control of Microorganisms by Physical and Chemical Agents © The McGraw−Hill Companies, 2002 tumor viruses, and recombinant DNA. They are also employed in re￾search labs and industries, such as the pharmaceutical industry, when a sterile working surface is needed for conducting assays, preparing media, examining tissue cultures, and the like. Radiation The types of radiation and the ways in which radiation damages or destroys microorganisms have already been discussed. The practical uses of ultraviolet and ionizing radiation in sterilizing objects are briefly described next. Radiation and its effects on mi￾croorganisms (pp. 130–31) Ultraviolet (UV) radiation around 260 nm (see figure 6.17) is quite lethal but does not penetrate glass, dirt films, water, and other substances very effectively. Because of this disadvantage, UV radiation is used as a sterilizing agent only in a few specific situa￾tions. UV lamps are sometimes placed on the ceilings of rooms or in biological safety cabinets to sterilize the air and any exposed sur￾faces. Because UV radiation burns the skin and damages eyes, peo￾ple working in such areas must be certain the UV lamps are off when the areas are in use. Commercial UV units are available for water treatment. Pathogens and other microorganisms are de￾stroyed when a thin layer of water is passed under the lamps. Ionizing radiation is an excellent sterilizing agent and pene￾trates deep into objects. It will destroy bacterial endospores and vegetative cells, both procaryotic and eucaryotic; however, ioniz￾ing radiation is not always as effective against viruses. Gamma ra￾diation from a cobalt 60 source is used in the cold sterilization of antibiotics, hormones, sutures, and plastic disposable supplies such as syringes. Gamma radiation has also been used to sterilize and “pasteurize” meat and other food. Irradiation can eliminate the threat of such pathogens as Escherichia coli O157:H7, Staphylo￾coccus aureus, and Campylobacter jejuni. Both the Food and Drug Administration and the World Health Organization have approved food irradiation and declared it safe. A commercial irradiation plant operates near Tampa, Florida. However, this process has not yet been widely employed in the United States because of the cost and concerns about the effects of gamma radiation on food. The U.S. government currently approves the use of radiation to treat poultry, beef, pork, veal, lamb, fruits, vegetables, and spices. It will proba￾bly be more extensively employed in the future. 1. Define thermal death point (TDP), thermal death time (TDT), decimal reduction time (D) or D value, z value, and the F value. 2. Describe how an autoclave works. What conditions are required for sterilization by moist heat, and what three things must one do when operating an autoclave to help ensure success? 3. How are pasteurization, flash pasteurization, ultrahigh temperature sterilization, and dry heat sterilization carried out? Give some practical applications for each of these procedures. 4. How can low temperature be used to control microorganisms? 5. What are depth filters and membrane filters, and how are they used to sterilize liquids? Describe the operation of a biological safety cabinet. 6. Give the advantages and disadvantages of ultraviolet light and ionizing radiation as sterilizing agents. Provide a few examples of how each is used for this purpose. 144 Chapter 7 Control of Microorganisms by Physical and Chemical Agents Figure 7.6 A Laminar Flow Biological Safety Cabinet. (a) A technician pipetting potentially hazardous material in a safety cabinet. (b) A schematic diagram showing the airflow pattern. Exhaust HEPA filter Motor/blower Supply HEPA filter Special light and electrical compartment High-velocity air barrier Optional support stand Safety glass viewscreen (b) (a)