熱殺菌の動力学にかんする2,3の考察 : 微生物の熱死滅にかんする動力学的研究-I

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The kinetics of thermal death of microbial cells, especially of bacterial spores, have been discussed deducing some equations.(I) Historicity of the systems concerned.The systems to be dealt with are classified into two large groups : (a) nonhistorical system and(b) historical system. In the former system, the heat sensitivity of cellular entities (or sensitive molecules), whose destruction is the cause of thermal death, is regarded to be constant throughout the time of heat treatment (Fig. 1). This system can be devided into the following subgroups : (a-1) single-hit type ; (a-2) multiple-hit type ; (a-3) the type of distributed sensitivity (cf. Fig. 2 and the dotted curve in Fig. 1 illustrating the course of thermal death when the sensitivity, k-value, is distributed normally among the population) ; others. In the historical system, the sensitivity is affected by heat to which spores (or survivors) have been exposed.(II) Formulation of the thoretical thermal death time (TDT) in the case of the nonhistrical system of single-hit type.Thermal death rate constant is regarded as the probability density of the event (thermal death), that is the probability per cell per unit time; the meaning is interpreted by Eq. (1), (2) and (3). The reciprocal (1/k) means therefore the most probable length of time while a cell survives in a single cell system. Statistical consideration by applying Poisson's process and the formula for calculating statistical weight leads to the deduction that 1/kN (≡τ_N) is the most probable time "necessary" for reducing the cell number (N) by one at a defined temperature [cf. Eq. (6)〜(9')]. The TDT(time just necessary for killing the last survivor starting with a sample containing N_0 cells (θ__<N_u-0>) can be calculated theoretically by summing up the τ_N-values for all the events [Eq. (10) and (11)]. When N_0 is large enough, Euler's number is applied in computing (θ__<N_u-0>) in Eq. (12).It is obvious from the above that τ_N is different from the time required for obtaining the state (N-1) "at the maximum frequency" starting from the state (N). But the above two values will be practically equal if N is large enough.If one applies the theoretical TDT for sterilizing a large number of samples containing a large number (N_0) of cells in each, about 57% of the samples will become perfectly sterile as calculated by Eq. (14). The relationship of temperature to the theoretical TDT-value is expressed in Eq. (15).(III) Some examples of historical systems.For simplicity, some examples of the historical systems of single-hit type are dealt with. In making discrimination between historical and nonhistorical systems, kinetic and physiological analyses are required since the change in slope of a log N curve at a constant temperature does not necessarily mean the change of heat sensitivity of the entity (sensitive molecule) in the course of sterilization [cf. multiple-hit (m', n') type shown in Fig. (1)]. Well-known heat activation of bacterial spores toward thermal inactivation is an example along this line. If viable spores are carefully counted without overlooking potential viability which would be visualized by appropriate pretreatment, the transformation of the entity can be observed as the sensitivity changes from low to high. In contrast to the above, spores of certain bacteria exhibit the change of sensitivity from high to low during heat treatment (heat-desensitization) at a constant temperature.The transformation of the entity between S and R having sensitivity of k_1 and k_2,respectively, is presumed in the above examples [cf. Fig. (3)]. Spores of Bacillus Pumilus behave according to the latter model. That the system concerned is historical has been assured through pre-incubation experiments, and the survival rate equation (20) is derived from Eq. (17)〜(19). The kinetic parameters can be estimated by analyzing thermal death rate with spores which have been incubated at various sublethal tempe
1966-08-25

熱殺菌の動力学にかんする2,3の考察 : 微生物の熱死滅にかんする動力学的研究-I

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