Summary: strains B Hill(FIL+) and B/r(FIL-), when grown on minimal medium containing glycerol, ceased respiration for several hours after ultraviolet (u.v.) irradiation at 520 ergs/mm. Treatment of cells with 5-fluorouracil (FU) had two effects on viability: (i) pre-irradiation treatment enhanced survival of B Hill(FIL+) but not that of B/r(FIL-) when survival was measured immediately after irradiation; (ii) pre- or postirradiation treatment of either strain resulted in a large and rapid increase in viability over the first hour of postirradiation incubation. Postirradiation treatment with pantoyl lactone (PL) resulted in a moderate increase in viability, but maintenance of growth and respiration was favoured only by FU. Postirradiation treatment with FU and PL together produced kinetics of respiration, growth and viability intermediate between those observed when the drugs were used separately. Irradiated B Hill organisms, which usually form filaments after u.v. irradiation, only did so in glycerol medium when FU was present. If FU was then replaced by PL, the filaments broke up and viability increased. Viability of B Hill was always higher when irradiated bacteria were plated on agar containing PL than without PL. Thus the increased viability caused by PL and FU when present singly during postirradiation incubation occurred by different means: PL by stimulation of division, and FU by maintenance of respiration.


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