Factors Determining the Rate of Mortality of Bacteria Exposed to Alkalinity and Heat

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Abstract
When vegetative cells (Escherichia coli) are exposed to the toxic action of NaOH, reduction appears to proceed at a regular logarithmic rate and does not exhibit the increasing mortality rate with progress of time, observed by Levine and his associates for spores. Rate of mortality in a cell population exposed to NaOH or to heat varies markedly with 2 factors: (a) Age of source-culture used for inoculation: Increasing the age of the source-culture from 8 to 11 hrs., decreases K (velocity, coefficient of rate of death) by 42%; a further increase to 14 hrs. decreases it again by 30% of the 11-hr. value; a further increase to 17 hrs. decreases K again by 11% of its 14-hr. value. The K for a 17-hr. culture is only a little over 1/3 that for an 8-hr. culture. (b) Concentration of inoculum: Exps. starting with inocula of 1-50 million cells per cc. show values of K 2-3 times higher than those starting with inocula of over 100 million cells per cc. The influence of concentration of cells used in the inoculum is attributed to production by the cells of substances which form a protective zone about each cell and which will obviously remain more concentrated when in the neighborhood of similar zones surrounding adjacent cells. In the case of heat-killing, it is suggested that the zone of protective substance in the neighborhood of a cell reacts on the cell wall to make it more resistant to the influence of heat.

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