INACTIVATION RATE OF BACILLUS SUBTILIS SPORES IRRADIATED BY ULTRAVIOLET RAYS

The UV inactivation of Bacillus subtilis spores, widely used for examining sterility, was kinetically studied to obtain fundamental information on the design and performance evaluation of practical sterilizers irradiated by ultraviolet rays. The inactivation rate at low extent of sterilization (survival fraction of more than 10-2) was determined under monochromatic irradiation and analyzed by the single-hit-killing model based on the target theory. The inactivation cross-section, φa, was strongly dependent on wavelength; the target number, m, was rather constant from 240 through 300 nm. The inactivation characteristics at high extent of conversion (survival fraction of less than 10-3) under polychromatic irradiation were simulated by considering the presence of a small portion of UV-resistant spores. An improved kinetic model was derived on the assumption of coexistence of two types of spores with different inactivation cross-sections and with the same target number. The observed wavelength-averaged inactivation cross-section for the major portion of spores was evaluated from both the inactivation cross-section obtained with monochromatic irradiation and the spectral distribution of light emitted from a polychromatic lamp.