Induction of lambda prophage near the site of focused UV laser radiation

DNA damage from photon scatter or beam spread during UV excimer laser irradiation was investigated using the induction of bacteriophage lambda in E. coli BR339. Prophage induction in these cells leads to the production of beta-galactosidase which can be detected colorimetrically by the application of appropriate substrates. An agar surface overlayed with BR339 cells was placed at various distances from the focal point of a converging lens and exposed to either 193 or 248 nm laser radiation. Energy densities ranging from approximately 5 mJ/cm2 to 30 J/cm2 were used. Ablation with 193 nm laser radiation produced an 800 microns wide clear 'trench' surrounded by a 500 microns zone of cells in which lambda had been induced. Following ablation with 248 nm laser radiation, the zone of induction was several millimeters wide. Exposures to 193 nm radiation at 170 mJ/cm2/pulse produced visible ablation of the agar surface at 1.7 J/cm2. Lambda induction was observed surrounding cleared ablation areas. The presence of induction in this system suggests that both 248 and 193 nm excimer laser radiation delivered at high energy densities has sufficient spread or scatter to damage DNA in cells surrounding areas of ablation.