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The unique energy transfer characteristics of swift cluster ions have attracted the attention of many researchers working on the analysis or processing of material surfaces, but the effects on living organisms remain unclear. We irradiated B. subtilis spores with monomer and cluster proton beams and examined their lethality; the 2 MeV H2+ shows a clearly lower lethality than 340 keV H+, even though both have a comparable linear energy transfer. The 2 MeV H2+ dissociates into a pair of 1 MeV H+ by losing the bonding electrons at the target surface. The estimated internuclear distance and the radial dose distribution suggest that the spread of deposited total energy over two areas separated by just several nanometers greatly diminishes beam lethality and that the energy density in the very center of the trajectory, possibly within a 1 nm radius, has a great impact on lethality. We also performed a whole genome resequencing of the surviving colonies to compare the molecular nature of mutations but failed to find a clear difference in overall characteristics. Our results suggest that cluster beams may be a useful tool for understanding biological effects of high linear energy transfer radiation.

Mutations caused by ion beams have been well-studied in plants, including ornamental flowers, rice, and algae. It has been shown that ion beams have several significantly interesting features, such as a high biological effect and unique mutation spectrum, which is in contrast to low linear energy transfer (LET) radiation such as gamma rays. In this study, the effects of double strand breaks and 8-oxo-2′-deoxyguanosine (8-oxodG) caused by ion-beam irradiation were examined. We irradiated repair-gene-inactive strains rad52, ogg1, and msh2 using carbon ion beams, analyzed the lethality and mutagenicity, and characterized the mutations. High-LET carbon ion-beam radiation was found to cause oxidative base damage, such as 8-oxodG, which can lead to mutations. The present observations suggested that nucleotide incorporation of oxidative damage gave only a limited effect on cell viability and genome fidelity. The ion-beam mutations occurred predominantly in 5′-ACA-3′ sequences, and we detected a hotspot at around +79 to +98 in URA3 in wild-type and mutant lines, suggesting the presence of a mutation-susceptible region.

Cluster ion beams have unique features in energy deposition, but their biological effects are yet to be examined. In this study, we employed bacterial spores as a model organism, established an irradiation method, and examined the lethal effect of 2 MeV C, 4 MeV C2, and 6 MeV C3 ion beams. The lethal effect per particle (per number of molecular ions) was not significantly different between cluster and monomer ion beams. The relative biological effectiveness and inactivation cross section as a function of linear energy transfer (LET) suggested that the single atoms of 2 MeV C deposited enough energy to kill the spores, and, therefore, there was no significant difference between the cluster and monomer ion beams in the cell killing effect under this experimental condition. We also considered the behavior of the atoms of cluster ions in the spores after the dissociation of cluster ions into monomer ions by losing bonding electrons through inelastic collisions with atoms on the surface. To the best of our knowledge, this is the first report to provide a basis for examining the biological effect of cluster ions.