7.Control of 38 Chapter 7 Control of Microorganisms by Physical and Chemical Agen ontrast,dis cion is the killing.inhibitio or n 7.2 The Pattern of Microbial Death lso ntially re A microbial po when ex sed to popul ion growth.is gen ores and a If the logarithm of the population number remaining is plotted d to levels that are con a straight line plot will result(compare figure 7.with figure When the the microorganism. on of infection n6edio t not d ctants If these ag ts are removed.resume.Their namesend in-static mpl of thei the total microbal populio not just to affect pathogen levels. s quite important in many situations. example the D value is I minute.The data are from table 7. Table 7.1 A Theoretical Microbial Heat-Killing Experiment Minute End of Minute Log of Survivors 9x10 00000 09Prescott−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 contrast, disinfection is the killing, inhibition, or removal of mi￾croorganisms that may cause disease. The primary goal is to de￾stroy potential pathogens, but disinfection also substantially re￾duces the total microbial population. Disinfectants are agents, usually chemical, used to carry out disinfection and are normally used only on inanimate objects. A disinfectant does not necessarily sterilize an object because viable spores and a few microorganisms may remain. Sanitization is closely related to disinfection. In san￾itization, the microbial population is reduced to levels that are con￾sidered safe by public health standards. The inanimate object is usually cleaned as well as partially disinfected. For example, sani￾tizers are used to clean eating utensils in restaurants. It is frequently necessary to control microorganisms on liv￾ing tissue with chemical agents. Antisepsis [Greek anti, against, and sepsis, putrefaction] is the prevention of infection or sepsis and is accomplished with antiseptics. These are chemical agents applied to tissue to prevent infection by killing or inhibiting pathogen growth; they also reduce the total microbial population. Because they must not destroy too much host tissue, antiseptics are generally not as toxic as disinfectants. A suffix can be employed to denote the type of antimicrobial agent. Substances that kill organisms often have the suffix -cide [Latin cida, to kill]: a germicide kills pathogens (and many non￾pathogens) but not necessarily endospores. A disinfectant or an￾tiseptic can be particularly effective against a specific group, in which case it may be called a bactericide, fungicide, algicide, or viricide. Other chemicals do not kill, but they do prevent growth. If these agents are removed, growth will resume. Their names end in -static [Greek statikos, causing to stand or stopping]—for ex￾ample, bacteriostatic and fungistatic. Although these agents have been described in terms of their effects on pathogens, it should be noted that they also kill or in￾hibit the growth of nonpathogens as well. Their ability to reduce the total microbial population, not just to affect pathogen levels, is quite important in many situations. 1. Define the following terms: sterilization, sterilant, disinfection, disinfectant, sanitization, antisepsis, antiseptic, germicide, bactericide, bacteriostatic. 7.2 The Pattern of Microbial Death A microbial population is not killed instantly when exposed to a lethal agent. Population death, like population growth, is gen￾erally exponential or logarithmic—that is, the population will be reduced by the same fraction at constant intervals (table 7.1). If the logarithm of the population number remaining is plotted against the time of exposure of the microorganism to the agent, a straight line plot will result (compare figure 7.1 with figure 6.2). When the population has been greatly reduced, the rate of killing may slow due to the survival of a more resistant strain of the microorganism. 138 Chapter 7 Control of Microorganisms by Physical and Chemical Agents 6 Minutes of exposure Log10 number of survivors 0 1 2 3 4 5 6 7 0 –1 5 4 3 2 1 D121 Figure 7.1 The Pattern of Microbial Death. An exponential plot of the survivors versus the minutes of exposure to heating at 121°C. In this example the D121 value is 1 minute. The data are from table 7.1. Table 7.1 A Theoretical Microbial Heat-Killing Experiment Microbial Number Microorganisms Killed Microorganisms Minute at Start of Minutea in 1 Minute (90% of total)a at End of 1 Minute Log10 of Survivors 1 106 9 × 105 105 5 2105 9 × 104 104 4 3 104 9 × 103 103 3 4 103 9 × 102 102 2 5 102 9 × 101 10 1 6 101 9 10 7 1 0.9 0.1 –1 a Assume that the initial sample contains 106 vegetative microorganisms per ml and that 90% of the organisms are killed during each minute of exposure. The temperature is 121° C