Transient frailty induced by cell division. Observation, reasons and implications

We know that stress-factors, e.g. X-rays, have an effect on cells that is more lethal in rapid exponential growth than in stationary phase. It is this effect which makes radiotherapy effective in cancer treatment. This stress effect can be explained in two ways: (a) more vulnerability in the growth phase, (b) improved protection capacity and repair mechanisms in the stationary phase. Although the two explanations do not exclude each other, they are very different in the sense that (a) is a general mechanism whereas (b) is strain and stress-factor dependent. In this paper we explore major facets of (a). Firstly, we emphasize that (a) can account for known experimental stress-factor evidence. Secondly, we observe that (a) rightly predicts that slow exponential growth (meaning with a doubling time of several hours) results in a lower death rate than fast exponential growth (doubling time of a fraction of one hour), an effect that cannot be explained in the (b) framework because both organisms are in the same phase. Thirdly, we conjecture that the stress-factor effect can be extended to situations of chronic stress due to non-optimal environmental conditions. If correct, this conjecture would imply that even in normal culture conditions the natural death rate is lower in the stationary phase than in the growth phase. Finally, the paper closes with the description of several open questions and of appropriate test-experiments meant to address them.