Resistance of spores of Clostridium botulinum 33A to combinations of ultraviolet and gamma rays.

Spores of Clostridium botulinum 33A exhibit a sigmoidal survival curve if subjected to gamma radiation. The present investigation was concerned with two questions: (i) what is the form of an ultraviolet (UV)-survival curve and (ii) what is the combined effect of UV- and gamma radiation? The UV-survival curve was found to be of sigmoidal type with a "shoulder" width of 675 ergs/mm(2) and a D(10) (exp) of 2,950 ergs/mm(2). To test the combination effect, spores were subjected to UV doses of 225, 450, 675, and 900 ergs/mm(2) followed by a series of increasing doses of gamma rays from 200 to 2,000 krad in 200-krad steps. The gamma ray-survival curves showed that increasing UV pretreatment caused a gradual loss of the "Prodiginine" yielding straight line exponential survival curves after preirradiation with UV doses of 675 ergs/mm(2) and above. Simultaneously the D(10) value for gamma-ray irradiation was reduced, e.g. UV preirradiation with 900 ergs/mm(2) reduced the D(10) by 40%. This observation emphasizes the potential practical advantage of combining UV and gamma rays for sterilization of heat-sensitive commodities.