Inactivation of Bacillus subtilis spores on packaging surfaces by u.v. excimer laser irradiation

Ultraviolet (u.v.) laser irradiation has been used to inactivate Bacillus subtilis spores deposited on to planar aluminium- and polyethylene-coated packaging surfaces. Kill kinetics were found to be diphasic, with an initial rapid inactivation phase followed by tailing. Although no definitive evidence was obtained, it is thought that spores located within packaging crevices/pores were primarily responsible for the observed tailing. Surviving spores were also found on the unexposed underside of cards and, to a lesser extent, within clumps. The log count reduction in B. subtilis was dependent on spore loading and total u.v. dose. In comparison, packaging surface composition, fluence (2-18 Jm-2) and frequency (40-150 Hz) had only a negligible effect. By irradiating boards carrying 106 spores, with a dose of 11.5 J cm-2, a log count reduction >5 was obtained. The mode of spore inactivation was primarily through DNA disruption. This was confirmed by the high sensitivity of spores lacking protective, small, acid-soluble proteins, in addition to the high frequency of auxotrophic and asporogenous mutations found amongst survivors.