The response of Bacterium lactis aerogenes when held at growth temperature in the absence of nutriment: an analysis of survival curves

Abstract

Suspensions of logarithmic phase cells of Bacterium lactis aerogenes at various cell densities held at growth temperature in the absence of added nutriment show a survival which is maximum at a particular cell density. At densities above and below this critical cell density survival is proportionally less. The present research studies the improvement in survival which occurs as the initial cell density is raised from a low value up to the critical value. At a density of 10 5 or 10 6 cells/ml. the death rate is very great and the suspension becomes sterile within several hours. At a density of 10 7 cells/ml. the death rate shows fluctuations—as a result of periodic waxing and waning in viable cell count. At the critical density, 10 8 cells/ml., an initially rapid death gradually diminishes until it eventually becomes negligible. Thus the survival curve has a ‘tail’. These observations have been explained as follows: logarithmic phase cells in the absence of food die as the result of loss of cell substance. The concentration of this cell effluent increases with time. At low densities the effluent never becomes sufficiently concentrated to be recoverable, so none of the cells survive. But at higher densities the effluent, as it accumulates from the larger number of cells, eventually becomes sufficiently concentrated to sustain those cells still viable at that time. Thus, the apparent anomalies in survival are shown to arise from an interaction between cells. These findings are discussed with regard to cell survival in general.