Exponential inactivation

Fig. 1 Exponential inactivation of micro-organisms (the survival curve).

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. 

For simplicity, the kinetics of exponential inactivation of populations of microorganisms will be illustrated by reference to the application of some form of inimical ireaiinent to a pure culture of bacteria. There are plenty of data in the scientific literature to support the general case. 

In summary, there is always a finite probability of a survivor occurring, no matter the strength or duration of a sterilization process. In other words, absolute sterility, in the sense of total freedom from all viable life forms, can never be achieved in practice. The acceptance of exponential inactivation lunetics has led to two different approaches to the establishment of standards of satisfactory sterilization treatment, the inactivation factor approach and the sterility assurance level (SAL) approach. 

When validation depends on extrapolation, it is of utmost importance to have confidence in the base data from which the extrapolation is to be made. The determinants of sterility assurance are defined by an equation describing exponential inactivation. When time is the variable, as in Fig. 2b, the expression is written as... 

Hie AAMI B2 Method The AAMI B2 method assumes far less than the AAMI BI method. It assumes exponential inactivation. It does not assume any standard but arbitrary distribution and it does not require estimation of bioburden. It docs, however, continue to require well controlled sterility testing meth-... 

Thermal damage to biological systems is caused by absorption of heat energy. Well-controlled laboratory studies show that when pure cultures of microorganisms are held in saturated steam at a constant sterilizing temperature there arc linear relationships between the logarithm of the number of survivors and the time of exposure (exponential inactivation). 

Preservative molecules are used up as they inactivate microorganisms and as they interact non-specifically with the significant quantities of contaminant dirt also introduced during use. This will result in a progressive and exponential decline in the efficiency of the remaining preservative. Preservative capacity is a term used to describe the cumulative level of contamination that a preserved formulation is likely to cope with before becoming so depleted as to become ineffective. This will vaiy with preservative type and complexity of the formulation. 

Figure 1 Absorbed dose necessary to produce a given biological effect on a given system, in the present case 50% inactivation of V79 hamster cells in exponential growth phase. The dose needed depends on the type of the particles (photons, alpha particles, or uranium ions) and on their energy. (From Ref 15.)...

If the rate of metabolite formation can be determined over a time period that is sufficiently short that significant enzyme inactivation does not occur (fccat > inacl), then the exponential term in Eq. (12) approaches unity and may be ignored. Equation (12) illustrates that the apparent Vmax for formation of a metabolite will decline as the incubation time increases when simultaneous enzyme inactivation occurs (Fig. 4). 

Experiments in which bacteria were illuminated at two different growth states, stationary and exponential phase and two different generations were performed (Fig. 17.5). Bacteria collected from the culture at their stationary phase were less readily inactivated in the presence and absence of Ti02 than those taken at exponential growth phase. This difference is highlighted when Ti02 is not added. 

In contrast to sterilization, the aim of disinfection techniques is to reduce the number of pathogenic bacteria, viruses, protozoa and spores by inactivation, so that the infectivity of the material is reduced drastically. During disinfection processes, the inactivation of microorganisms follows an exponential decay according to Eq. 9-1. 

Although these reactions are complex at a biochemical level, their kinetics approximate to reactions of the first order. Thus, the kinetics of inactivation of populations of pure cultures of micro-organisms take the typical exponential form of reactions of the first order. What this means in experimental practice is that there is a linear relationship when numbers of microorganisms held at high temperatures are plotted on a logarithmic scale against time plotted on an arithmetic scale (Fig. 1). 

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... 

Even though the definition of sterility is an absolute condition, the effectiveness of the sterilization process can be determined by measuring the reduction of microbial population. Such measurements reveal the kinetics of microbial inactivation, and it is from the exponential nature of inactivation that the concept of sterility assurance level (SAL) is derived. This value... 

Since the inactivation of bacteria follows an exponential decay process with a limiting value tending towards zero, the absolute sterility can never be obtained. The D,g value is the absorbed dose required to reduce a microbial population to 10% of its initial value, so that in industrial applications, a SAL value of 10" is reached after 3 x D value. For drugs, a dose of 25 kGy (or kJ kg ) is generally higher than 6 x D,g value and achieves the minimum SAL required of 10 . Many reviews demonstrated that for drugs with low bioburdens (initial contamination by microorganisms), sterilization was achieved with doses even lower than 15 kGy. 

Palou et al. (1998) used the equation of Fermi to demonstrate that the critical pressure for Z. bailii is 297.8 MPa for cells in sfationary phase and 145 MPa for the exponential phase at = 0.98. However, this model is only used to describe and compare the inactivation parameters between microorganisms that have been subjected to the same treatment. Other models that can be applied are those commented by Alzamora et al. (2005). 

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