【文章內(nèi)容簡介】 This fact has made it necessary to develop special qualifications(指標(biāo) ) that define and delineate（描述） microbial death. Factors That Affect Death Rate ? The ability to define microbial death has tremendous theoretical and practical importance. It allows medicine and industry to test the conditions required to destroy microanisms, to pinpoint（探明） the ways that antimicrobial agents kill cells, and to establish standards of sterilization and disinfection in these fields. ? The cells of a culture show marked variation in susceptibility （敏感性） to a given microbicidal agent. Death of the whole population is not instantaneous but begins when a certain threshold（拐點(diǎn)） of microbicidal agent is met and continues in a logarithmic（對數(shù)的 ） manner as the time or concentration of the agent is increased (). ? The effectiveness of a particular agent is governed by several factors besides time. These additional factors influence the action of antimicrobial agent （殺菌劑） . 1. The number of microanisms ().A higher load of contaminants requires more time to destroy. 2. The nature of the microanisms in the population (). In most actual circumstances of disinfection（消毒 ） and sterilization, the target population is not a single species of microbe but a mixture of bacteria, fungi, spores, and viruses, presenting an even greater spectrum of microbial resistance. 3. The temperature and pH of the environment. 4. The concentration (dosage, intensity) of the agent. For example, UV radiation is most microbicidal at 260 nm。 most disinfectants are more active at higher concentrations. 5. The mode of action of the agent (). How does it kill or inhibit the microanism? 6. The presence of solvents（有溶解力的） , interfering anic matter, and inhibitors. Large amounts of saliva, blood, and feces can inhibit the actions of disinfectants and heat. HOW ANTIMICROBIAL AGENTS WORK: THEIR MODES OF ACTION ? An antimicrobial agent’ s adverse（不利的） effect on cells is known as its mode of action. Agents affect one or more cellular targets, inflicting damage progressively until the cell is no longer able to survive. Antimicrobials have a range of cellular targets, with the agents that are least selective in their targeting tending to be effective against the widest range of microbes. ? The cellular targets of physical and chemical agents fall into four general categories: 1. the cell wall. 2. the cell membrane. 3. cellular synthetic processes (DNA,RNA), and 4. proteins. The Effects of Agents on the Cell Wall ? The cell wall maintains the structural integrity of bacterial and fungal cells. Several types of chemical agents damage the cell wall by blocking its synthesis, digesting it, or breaking down its surface. A cell deprived of a functioning cell wall bees fragile(脆弱的 ) and is lysed（溶解） very easily. How Agents Affect the Cell Membrane ? All microanisms have a cell membrane posed of lipids and proteins, and we learned in previous chapter, a cell’ s membrane provides a twoway system of transport. If this membrane is disrupted, a cell loses its selective permeability（透過性 ） and can neither prevent the loss of vital molecules nor bar the entry of damaging chemicals. Agents That Affect Protein and Nucleic Acid Synthesis ? Microbial life depends upon an orderly and continuous supply of proteins to function as enzymes and structural molecules. As we saw in chapter 9, these proteins are synthesized on the ribosomes through a plex process called translation. For example, the antibiotic choramphenicol（氯霉素） binds to the ribosomes of bacteria in a way that stops peptide bonds from forming proteins required in growth and metabolism and are thus inhibited from multiplying. ? The nucleic acids are likewise necessary for the continued functioning of microbes. Some agents bind irreversibly to DNA, preventing both transcription and translation。 others are mutagenic agents. Gamma, ultraviolet, or X radiation causes mutations that result in permanent inactivation of DNA. Chemicals such as formaldehyde（甲醛 ） and ethylene（乙烯 ） oxide also interfere with DNA and RNA function. Practical Concerns in Microbial Control ? Numerous considerations govern selection of a workable method of microbial control. These are among the most pressing concerns: 1. Does the application require sterilization, or is disinfection adequate? In other words, must spores be destroyed, or is it necessary to destroy only vegetative pathogens? 2. Is the item to be reused or permanently discarded? If it will be discarded, then the quickest and least expensive method should be chosen. 3. If, it will be reused, can the item withstand heat, pressure, radiation, or chemicals? 4. Is the control method suitable for a given application? (For example, ultraviolet radiation is a good sporicidal（殺孢子的 ） agent, but it will not perate solid materials.) Or, in the case of a chemical, will it leave an undesirable residue（殘基 ） ? 5. Will the agent perate to the necessary extent? 6. Is the method costand laborefficient, and is it safe? ? A remarkable variety of substances can require sterilization. They run the gamut（整個(gè)領(lǐng)域 ） from durable solids such as rubber to sensitive liquids such as serum（血清 ） , and form air to tissue grafts（移植） . ? Considerations such as cost, effectiveness, and method of disposal are all important. For example, the disposable plastic items such as catheters（導(dǎo)管 ） and syringes（注射器 ） that are used in i