Survivor/time curve

Inspection of the death curves obtained from viable count data had early ehcited the idea that because there was usually an approximate, and under some circumstances a quite excellent, linear relationship between the logarithm of the number of survivors and time, then the disinfection process was comparable to a unimolecular reaction. This imphed that the rate of killing was a function of the amount of one of the participants in the reaction only, i.e. in the case of the disinfection process the number of viable cells. From this observation there followed the notion that the principles of first-order... 

The resistance of an organism to a sterilizing agent can be described by means of the D-value. For heat and radiation treatments, respectively, this is defined as the time taken at a fixed temperature or the radiation dose required to achieve a 90% reduction in viable cells (i.e. a 1 log cycle reduction in survivors Fig. 20.2k). The calculation of the D-value assumes a linear type A survivor curve (Fig. 20.1), and must be corrected to allow for any deviation from linearity with type B or C curves. Some typical D-values for resistant bacterial spores are given in Table 23.2 (Chapter 23). 

Figure 4 Survivor curves showing the effect of decreasing the microbial load (A) from 106 to 102 on the time required to achieve a probability of nonsterility (B) of 10 6.

Let us suppose that the inimical process can be applied to a population of bacteria in an incremental manner. Initially and after various periods of exposure we can withdraw samples from the population and count the number of viable bacteria present. When we plot these data as a survival curve on arithmetic axes we get the form of Fig. 2a. The relationship of survivors to time of exposure shows an initial rapid decline and then levels out and becomes asymptotic to the time axis. If we were to convert the survivor data to logarithms and then plot these logarithmic data against time of exposure we would obtain a straight-line relationship (Fig. 2b). This is the general form for exponential or logarithmic survival curves. 

The kinetics of inactivation of microbial populations exposed to ethylene oxide are exponential [2] when the logarithm of the number of survivors is plotted against time with all other factors (e.g.. gas concentration, humidity, temperature) held constant. Shouldered curves have been occasionally noted instances of tailed" inactivation kinetics have been ascribed to clumping or environmental protection. Good experimental data are not easily obtained. Experimental design should concentrate on rapid attainment of the gas concentration intended. Inactivation from residual sierilant may also lead to misleading results. 

The possible fate of 1,000 atoms followed by simulation. The probability that a given atom will survive a single time unit has been fixed at p = 0.9. The number of survivors decreases monotonically according to a step function that follows the curve of an exponential function. On the abscissa, the standard notation is used as recommended by lUPAP to express the physical quantity t as a product of a numerical value t and a unit [t] (See the text for more detail)... 

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