The Sensitivity of Escherichia Coli to Ionizing Particles of Different LETs

SummaryMeasurements have been made of the inactivation doses for Escherichia coli exposed to α-particles and protons of different LETs and also to γ-rays. These have been interpreted by target theory in terms of two types of lethal damage to the bacterial genome. Damage of type 1 affects only one strand of the DNA macromolecule and is partially reparable in vivo. Damage of type 2 following one track intersection of the DNA affects both strands and is irreparable. The identity of type 1 damage is uncertain; it does not appear to cause appreciable inactivation by producing ‘base’ mutations but may lead to a lesion recognizable in vitro as a single-strand break. Type 2 damage probably corresponds to double strand ‘scission’ of DNA as observed in vitro.