Apparent antimutagenic effect of ultraviolet irradiation

Strain SL3367 is a S. typhimurium LT2 hisG46 stock which spontaneously reverts to His+ at a high frequency. Plates of defined medium with 1% (v/v) nutrient broth inoculated with ca. 108 washed SL3367 cells were incubated, untreated or after UV irradiation. After 2 days at 370C, an average of 165 His+ colonies were obtained per control plate but significantly fewer, 105 His+ colonies, on plates irradiated at a fluence of 7 J/m2. The dry weight of bacteria in washings from plates incubated 14 h (by which time growth of His- cells had ceased) was the same for irradiated and non-irradiated plates but the yield of colony-forming units from irradiated plates was less than from control plates, by about the same factor as the reduction in yield of His+ colonies caused by the same fluence. Washings from incubated irradiated plates, but not those from control plates, contained long filaments as well as bacteria of normal size; on transfer to nutrient-agar slide cultures cells of normal size grew into microcolonies but filaments did not grow. The reduced plateau yield of viable His- cells caused by consumption of much of the growth-limiting supply of histidine by irradiated cells growing into non-viable filaments reduces the number of auxotrophic bacteria at risk for spontaneous reversion and so accounts for the apparent antimutagenic effect of UV irradiation. (orig./AJ)