Different criteria of death have been compared experimentally and quantitatively. Pure cultures of a yeast have been subjected to adverse conditions, and the number of dead cells, judged by different tests, has been determined in successive time intervals. The yeast cultures were exposed to heat, to HgCl2, to ultraviolet light, and to x-rays.
In each case, the cells lost the power of reproduction (measured by the plate count) most rapidly. The loss of fermentation (measured by the CO2 pressure) was less rapid. Still slower was the change in staining reaction with methylene blue, and the loss of selective permeability of the plasma membrane (measured by the percentage of cells staining with Congo red). Slowest of all was the coagulation of protoplasm as observed in the dark-field.
In the case of death by heat or by HgCl2, the rate of loss of reproduction was about twice as rapid as that of the loss of fermentation, about three times that of the loss of semipermeability, and about forty times as large as the rate of coagulation. With ultraviolet light and with x-rays, these ratios were decidedly different.
The technique employed does not permit the conclusion that any one criterion of death is the prerequisite for other criteria. It does not appear probable that loss of reproduction is the prerequisite for loss of fermentation or of semipermeability because the ratios of the velocities of these processes are not the same with all causes of death.
There is no evidence that cells may show certain criteria of death immediately after exposure, and recover later.