PROCESS OF MICROBIAL DESTRUCTION

Regardless of the type of lethality induced by a sterilization process—whether
it be heat, chemical, or radiation—micro-organisms, upon exposure to adequate
levels of such treatments, will die according to a logarithmic relationship between
the concentration or population of living cells and the time exposure or
radiation dose to the treatment.
A. D Value
The D value is a single quantitative expression of the rate of killing of microorganisms.
The D term refers to the decimal point in which microbial death
rates become positive time values by determining the time required to reduce
the microbial population by one decimal point. This is also the time required
for a 90% reduction in the microbial population. Hence, the time or dose it takes
to reduce 1000 microbial cells to 100 cells is the D value. The D value is
important in the validation of sterilization processes for several reasons.
1. It is a specific kinetic expression for each micro-organism in a specific
environment subjected to a specific sterilization agent or condition.
In other words, the D value will be affected by
a. The type of microorganism used as the biological indicator.*
b. The formulation components and characteristics (e.g., pH).
c. The surface on which the micro-organism is exposed (glass, steel,
plastic, rubber, in solution, dry powder, etc.).
d. The temperature, gas concentration, or radiation dose of the particular
sterilization process.*
2. Knowledge of the D value at different temperatures in heat sterilization
is necessary for the calculation of the Z value. (See p. 87.)
3. The D value is used in the calculation of the biological F value. (See
p. 87.)
4. Extrapolation of the D value from large microbial population values
to fractional (e.g., 10−x) values predicts the number of log reductions
a given exposure period will produce.
D values are determined experimentally by either of two methods, the
survivor-curve method or the fraction-negative method [7,8]. The survivor-curve
method is based on plotting the log number of surviving organisms versus an
independent variable such as time, gas concentration, or radiation dose. The
fraction-negative method uses replicate samples containing identical spore populations
treated in an identical manner and determining the number (fraction) of
samples still showing microbial growth after treatment and incubation. Fractionnegative
data are used primarily for determining D values of micro-organisms
exposed to thermal destruction processes. The following discussion concentrates
on D values calculated by the survivor-curve method.
Data obtained by the survivor-curve method are plotted semilogarithmically.
Data points are connected by least-squares analysis. In most cases the
equation used is the first-order death rate equation,
log N = a + bt (1)
where N is the number of surviving organisms of time t, a is the Y intercept,
and b is the slope of the line as determined by linear regression. The D value is
the reciprocal of the linear slope,
D = 1
b
(2)
Many micro-organisms produce nonlinear survivor curves, such as 1-B in Figure
1. The cause of nonlinear survivor curves has been explained by several theories,
such as the multiple critical sites theory [9], experimental artifacts [10],
and the heterogeneity of spore heat resistance [11]. Mathematical models for
concave survivor curves have been developed by Han et al. [12]. They are quite