Studies on induced variation in the rhizobia. II. Radiation sensitivity and induction of antibiotic-resistance markers.

The relation between survival and dose of ultraviolet (UV) light or X rays was, with only one exception, nonlinear for late-log-phase cultures of four species of Rhizobium. The ld(90) for different bacterial strains ranged from 400 to 3,000 ergs per mm(2) for UV treatment, and from 3.0 to 7.5 kr for X-ray treatment. Minor existing differences in response to some antibacterial compounds (disc tests) were usable as secondary markers for genetic experiments. Spontaneous and radiation-induced mutants, resistant to high levels of several antibiotics, notably dihydrostreptomycin and erythromycin, were isolated for primary marker purposes. A significant phenotypic lag for mutation to dihydrostreptomycin resistance was noted for UV-irradiated cells. The UV-induced frequency exceeded the X-ray-induced frequency by a factor of at least three, at a comparable level of lethality. No significant change in the symbiotic capability was observed in rhizobial strains marked with antibiotics resistance.