The disinfection process does not take place at once but is a gradual operation in which the number of organisms killed in unit time is greater at the beginning and becomes less and less as the exposure period is increased. If the numbers of survivors in unit volume are plotted against time, the points fall on smooth curves. If the logarithms of the numbers of bacteria surviving in unit volume are plotted against time, a straight line is obtained.
Anthrax spores were treated with 5 per cent phenol and incubated at 33⅓°C. The curved line represents the numbers of anthrax spores surviving in unit volume, and the straight line represents the logarithms of the concentration of survivors.
The disinfection process appears to follow the mass-action law and to proceed in accordance with the monomolecular equation, or a reaction of the first order, provided the disinfectant is present in large excess –
where N1 and N2 represent the number of surviving bacteria in unit volume after times t1 and t2, respectively.
Importance of Logarithmic Survivor Curves:
In the destruction of bacteria by various agents, the greater the number of cells present, the longer will be the time necessary for complete sterilization. This may be shown in the following example- A suspension contains 200,000 organisms per milliliter. If the bacteria are destroyed at the rate of 90 per cent per minute, the number of survivors at the end of 7 min. will be –
The last figure means that 2 living bacteria remain in 100 ml. of the suspension. If instead of 200,000 the suspension contains only 200 organisms per milliliter, the time required to reduce the count to 2 bacteria per 100 ml. will be only 4 min. –
The relationships between bacterial numbers and the times required to produce complete sterilization are of great value in the canning and dairying industries, in bacteriology, and in surgery.