Synergistic lethal action of ultraviolet-violet radiations and mild heat in Escherichia coli

The lethal interaction of far ultraviolet (254 nm), near ultraviolet (334 and 365 nm) and violet visible (405 nm) radiation treatment with mild heat treatment was studied. Except at 254 nm, a strong positive radiation dose-dependent interaction (synergism) was always observed. The efficiency of sensitisation to heat, as a function of dose at each wavelength was found to be directly correlated with the dose necessary to eliminate the shoulder from the survival curve of a repair proficient strain but was apparently unrelated to the relative near-ultraviolet sensitivities of a repair deficient strain. The interaction was independent of the order of treatments. A radiation dose of 106 Jm-2 at 365 nm slightly sensitised a cell population to 450C incubation (normally non-lethal) and strongly sensitised the cells to 480C treatment (normally 80 percent survival after 2 hours). It is proposed that in addition to DNA damage, both heat treatment and near ultraviolet treatment interfere with DNA recovery mechanisms so that the combination of the two agents inevitably leads to a strong positive interaction. (author)