Ultraviolet (193, 216 and 254 nm) Photoinactivation of Escherichia coli Strains with Different Repair Deficiencies

Escherichia coli K12 bacteria strains AB1157 (repair-deficient wild-type, <tex-math>${\rm uvrA}^{+}\ {\rm recA}^{+}$</tex-math>), AB1886 (<tex-math>${\rm uvrA}^{-}$</tex-math>), AB2463 (<tex-math>${\rm recA}^{-}$</tex-math>) and AB2480 (<tex-math>${\rm uvrA}^{-}\ {\rm recA}^{-}$</tex-math>) were exposed to 254 nm germicidal UV and 216 or 193 nm laser radiation. The mean lethal incident dose (<tex-math>$D_{37}$</tex-math>) for AB1157 does not change significantly with wavelength, whereas it increases for the other strains on going from <tex-math>$\lambda _{{\rm irr}}=254$</tex-math> to 193 nm, e.g. eightfold for AB2480. Quantum yields for inactivation, due to light absorbed by the chromosomal DNA, have been estimated from the <tex-math>$D_{37}$</tex-math> values. The large differences in <tex-math>$D_{37}$</tex-math> between <tex-math>${\rm uvrA}^{+}$</tex-math> and <tex-math>${\rm uvrA}^{-}$</tex-math> strains indicate a significant contribution of pyrimidine dimers and (6-4) photoproducts to photodamage of DNA in the cells. The formation of dimers even with <tex-math>$\lambda _{{\rm irr}}=193\ {\rm nm}$</tex-math> is supported by the result that the photoreactivable sector is larger than 63%. Inactivation of E. coli upon irradiation at 193 and 216 nm is therefore due to damage to intracellular DNA rather than to membrane or protein damage as is the case for mammalian cells. The ratio of the lethal doses for AB1157 vs AB2480 is ≈900 with <tex-math>$\lambda _{{\rm irr}}=254\ {\rm nm}$</tex-math>, but only 160 with <tex-math>$\lambda _{{\rm irr}}=193\ {\rm nm}$</tex-math>. We propose that this is due to the better repair of photodimers and (6-4) photoproducts than of damage induced by photoionization via upper excited states. Irradiation at 193 nm inactivates AB1157 mainly by damage due to photoionization and AB2480 by damage due to photodimers in the chromosomal DNA.