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Cells of E. coli B/r labelled with  32 P undergo inactivation during storage in liquid nitrogen (-196° C). Thawing the samples after storage and plating them immediately on complete medium yields colonies of uniform appearance. However, replica plating them on minimalmedium demonstrates, that during  32 P decay there occured a significant increase in nutritionally deficient (auxotrophic) mutants. When 10 -4  is the survival-rate, 0.67% of surviving cells give rise to auxotrophic colonies, 40 - 50% of which have prototrophic sectors. During storage the increase of mutants is linear with respect to inactivation of bacteria and to the fraction of  32 P decayed. The experiments carried out show, that inactivation and mutation are due to the decay of  32 P atoms incorporated in cellular structures and not do effects of freezing, toxicity of contaminants present in the  32 P solution, or ionization of β-electrons. An increase of mutants by positive selection in the course of inactivation was excluded experimentally

Mutagene Wirkung des Zerfalles von radioaktivem Phosphor nach Einbau in Zellen von Escherichia coli