Kinetics of biocide kill

The efficient use of biocides to control microbial contamination is dependent upon selecting the most potent agent at the anticipated end-use concentration. This is based upon an accurate determination of two basic parameters:

• 1.(1) The time taken by the biocide to achieve a total kill (death rate or decimal reduction time).
• 2.(2) The effect of biocide concentration on the death rate or decimal reduction time.

The time taken to achieve a total kill can be calculated from the death rate. In the simplest case a plot of the natural logarithm of survivors declines linearly when plotted against time and the slope of that line is the death rate. However, the plot of the line of survivors against time is frequently non-linear. Concave curves may result from attempts to control a mixed population with different degrees of tolerance to the biocide; the shape of the curve being a combination of two or more different linear declines. Convex curves, or curves with a shoulder, may be due to one of three phenomena. The target organisms may adhere together in clumps of two or more; the nature of the reaction of biocide with the target organism is one where the organism first changes from a resistant to a susceptible state; or the nature of the biocide molecule is such that uptake is relatively slow and death only commences when a critical concentration has accumulated within the cell. With all such convex curves the decline eventually becomes linear and again the slope is the death rate.The relationship between death rate and biocide concentration is rarely proportional and usually exponential. Thus halving the concentration may cause a disproportionate increase in the decimal reduction time. This relationship is controlled by the concentration coefficient or concentration exponent. When the logarithm of concentration is plotted against the log of death rate or log of decimal reduction time, a linear relationship is obtained whose slope is the concentration coefficient. Thus if the death rates at two or more concentrations are known, the concentration coefficient can be determined and the effect of dilution predicted.The mathematical interpretation of these phenomena and their influence on biocide treatment are discussed.