Abstract— –Lethality in a repairable strain (WP2) and an excision repair deficient strain (WP2hcr) of Escherichia coli was studied at wavelengths of 254, 313, 365, and 390–750 nm. Survival curves were empirically fitted to the expression S= 1 ‐ (1‐e‐kl)“, where S is the fraction surviving, D is the incident dose in ergs mm‐2, k is the inactivation constant in units of (erg mm‐2)‐1 and n is the ‘shoulder constant’. The repairable sector (k(hcr‐)–k(hcr‐)lk(hcr‐), a conservative estimate of the repair capability of E. coli WP2, was 0.91 at 254 nm, 0.92 at 313 nm, 0.60 at 365 nm, and 0.13 at 390–750 nm. Although there was no oxygen enhancement of inactivation at 254 nm and 313 nm, a strong enhancement was identified at 365 nm and 390–750 nm. These results suggest that oxygen‐dependent damage induced by near u.v. (365 nm) can be partially repaired by the excision‐repair system in E. coli.

Nutrient agar medium is made highly toxic to certain repair-deficient strains of Escherichia coli by exposure of the uninoculated plates to radiation from cool-white fluorescent lamps or black-light fluorescent lamps. This sensitivity is associated with the genetic deficiencies, fil, phr, and recA. Repair-sufficient and uvr strains are only slightly affected by the irradiated media. The poor growth and reduced plating efficiency frequently associated with BPhr and recA strains are very likely caused by inadvertent exposure of the medium to fluorescent light.

A vapor arc light source has been adapted to the study of the lethal action on bacteria of near-ultraviolet (UV) and visible light. Its use makes possible much shorter exposure times than could be obtained from previously available sources. The output of radiant energy is sufficient to provide a fairly detailed action spectrum for lethality in the long-UV and visible region without the addition of exogenous sensitizers. Populations of cells of Escherichia coli WP2 were inactivated through five log 10 cycles with light at 460 nm. Significant inactivation also was obtained with light at 550 and 650 nm.

A vapor arc light source has been adapted to the study of the lethal action on bacteria of near-ultraviolet (UV) and visible light. Its use makes possible much shorter exposure times than could be obtained from previously available sources. The output of radiant energy is sufficient to provide a fairly detailed action spectrum for lethality in the long-UV and visible region without the addition of exogenous sensitizers. Populations of cells of Escherichia coli WP2 were inactivated through five log10 cycles with light at 460 nm. Significant inactivation also was obtained with light at 550 and 650 nm.