Sterilization survival curve

Figures presented in various sections apply to experiments in which pure cultures have been irradiated in a controlled medium. In actual practice one is faced with the problem of producing an arbitrarily defined level of commercial sterility in an undefined medium with a mixed population. Commercial sterility can mean anywhere from 10 8 to 10 ia organism/g. depending on the product and type of contamination. Systematic data which would give the detailed shape of the survival curve in the region of commercial interests are nonexistent. The results of a number of experiments in which workers claimed to have achieved sterility are tabulated in Table VII. In nearly all cases insufficient data were obtained to allow a satisfactory statistical evaluation. Furthermore, differences and confusion in the dosimetry make the data rather approximate.

This exponential order of inactivation of microbial populations (Fig. 2, the exponential survival curve) has subsequently been demonstrated to be a general characteristic of microorganisms in all processes of sterilization. The logarithmic axis of the exponential survival curve has no zero point. Thus, there can be... 

Exponential inactivation is the basis of the concept of sterility assurance. If the behavior of microbial populations in response to a particular sterilizing procedure is regular and exponential over the region of the survival curve within which their response can be monitored, then the treatments required to achieve SALs of 10 can be extrapolated. 

The means of attaining sterility is called sterilization. In fact, this killing process must be rather considered as an inactivation. The death of a cell is indeed effective when it cannot reproduce itself. For this reason, the effect of a sterilization process can be followed by survival curves. 

The determinants of sterility assurance that must be considered in determining appropriate treatment levels to achieve particular SALs (in this text it will be assumed that the target SAL is lO ) or in validating existing treatment levels are, therefore, bioburden (microorganisms contaminating the item prior to treatment) and the shape and slope of the survival curve. 

Beside bioburden, the other determinant of sterility assurance is the survival curve and its shape and its slope. It is not correct to assume that all survival curves are of the simple linear type when data is plotted on semilogarithmic graph paper. Three general types of survival curve have been reported, the exponential curve, the shouldered" curve, and the "tailed" curve (Fig. 4). 

If it is assumed that the technology of the sterilization treatment in question can be controlled sufficiently well to deliver reasonably precise incremental treatment levels, then the procedures and precautions required to derive reproducible survival curves are quite similar. 

Doolan and coworkers [18] used three compendial recovery conditions to construct survival curves for naturally contaminated medical devices soybean casein digest medium incubated at 20-25 C, the same incubated at 30-35 C. and fluid ihioglycollatc medium incubated at 30-35 C for 14 days. Three tests for sterility therefore constituted the basis for observing the presence of viable microorganisms in three replicate conditions making up one sample. 

Tailed Survival Curves Tailed survival curves cannot be extrapolated. They are characterized by a slope that diminishes with increasing exposure to the sterilization treatment. They are often described as concave. ... 

Survival Curves with Mixed Cultures Although most published survival curves have been obtained in the laboratory using pure cultures of microorganisms, most real life sterilization processes must deal with mixed cultures, probably heterogeneously distributed over items. There are comparatively few published data on the behavior of mixed cultures, except as they have arisen by accident as contamination in lab( atory studies. There is no reason to believe that the different components within a mixed culture do not act completely independently of one another with respect to their responses to sterilization treatments. This tends to be a fundamental assumption in all approaches to validation of existing sterilization processes and of new process development. 

Figure 12-1. Survival curves for B. subtilis spores exposed to a N2/O2 diseharge afterglow (field frequeney is 2450 MHz). Gas pressure in the sterilization ehamber is 5 Torn Gas flow is one standard L/min with some percentage of oxygen added.

Contribution of the Electron Flux from Non-Thermal Plasma into Deactivation of Bioiogicai Micro-Organisms. Based on the equation (12-1) describing contribution of the non-thermal plasma electrons into sterilization, derive corresponding formula for the survival curve. Compare the curve related to contribution of plasma electrons with experimental survival curves for atmospheric-pressure discharges presented in Section 12.2. 

The term sterile , in a microbiological context, means no surviving organisms whatsoever. Thus, there are no degrees of sterility an item is either sterile or it is not, and so there are no levels of contamination which may be considered negligible or insignificant and therefore acceptable. From the survivor curves presented, it can be seen that the... 

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