ボツリヌスE型菌芽胞の放射線抵抗性に関する研究 (第3報)

Abstract

The serious problem of microbiological safety in radurization of fish is an earlier development of toxicity in low-dose irradiated samples of fish inoculated with spores of Clostridium botulinum type E than in unirradiated controls.The present investigation was designed to determine whether exposure of sublethal dose of γ-irraaiation to spores of this organism causes them to stimulate outgrowth and to produce more toxin in a sterilized medium.Spores of Clostridium botulinum type E were subjectedto a sublethal dose (300 Krad) of γ-irradiation, then inoculated into TPGY medium (trypticase, peptone, glucose, yeast extract) at an inoculum level of 104 spores/ml along with unirradiated controls. The cultures were incubatedat both 30°C and 10°C. Comparison of growth and toxin production by irradiated and unirradiated spores was made in each of the cultures.At 30°C, the growth rate during logarithmic phase of growth was initially higher in the culture incubated with irradiated spores than in the culture inoculated with unirradiated spores, resulting in a stimulatory effect of γ-radiation on the outgrowth of spores (Fig. 1). Toxin was detected in both the cultures after 9 hours of incubation though the titre in the former was 5 times as much as in the latter. The toxin production proceeded in parallel with each of the growth curves and reached the same titre at 33 hours of incubation.At 10°C, the results obtained were quite the oposite of those mentioned above. The growth and toxin production began to occur after 5 days of lag phase in both the cultures inoculated with irradiated and unirradiated spores, although the extent and rate were significantly higher in the latter than in the former (Fig. 2). The toxin titre, however, showed the same level after 8 days of incubation.The mechanisms involved in the stimulatory effect of low-dose γ-irradiation on the outgrowth and toxin production when incubated at 30°C are unknown, but regardless of an activation phenomenon in germinating spores. The delaying effect of γ-irradiation on the growth and toxin production when incubated at 10°C appears to be accounted for by the less recovery of spores survived at an unfavorable post-irradiation temperature.