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 s ubtilis spores deposited on to planar aluminium- and polyethylene-coated pa ckaging surfaces. Kill kinetics were found to be diphasic, with an initial rapid inactivation phase followed by tailing. Although no definitive eviden ce was obtained, it is thought that spores located within packaging crevice s/pores were primarily responsible for the observed tailing. Surviving spor es were also found on the unexposed underside of cards and, to a lesser ext ent, within clumps. The log count reduction in B. subtilis was dependent on spore loading and total u.v. dose. In comparison, packaging surface compos ition, fluence (2-18 Jm(-2)) and frequency (40-150 Hz) had only a negligibl e effect. By irradiating boards carrying 10(6) spores, with a dose of 11.5 J cm(-2), a log count reduction > 5 was obtained. The mode of spore inactiv ation 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 fo und amongst survivors.