Inactivation of B. cereus spores on agar, stainless steel or in water with a combination of Nd:YAG laser and UV irradiation

Abstract

Nd:YAG laser irradiation at 1.06 μm and germicidal ultraviolet irradiation (UV) were combined to treat B. cereus spores lawned on agar surfaces, stainless steel discs or distilled water. On the agar surface the applied laser energy density was 3000 J cm − 2 (applied as 100 or 200 W) and the UV (2 × 8 W at 24 cm) irradiance was 190 μW cm − 2 over 2 to 10 min. Laser treatment alone was insufficient to produce zones of clearing and the UV was only effective for exposures > 6 min, where confluent growth was slightly reduced. However, the combined treatment produced cleared zones that increased with UV exposure. The spores proved relatively resistant to laser treatment in distilled water with ∼0.5 log reduction in viability after exposure to 2400 J cm − 2 which raised the sample temperature to 90 °C. A multifactorial experiment was devised to investigate the interaction between the laser pre-treatment, the delay between treatments and the subsequent UV exposure. There was a significant decrease in spore viability for the interactions between the laser and UV treatment and the laser treatment and the delay time. From the results of this study it was deduced that the biocidal action of combined Nd:YAG laser and UV irradiation was additive and it provided an increased rate of inactivation of B. cereus spores. The ability to combine such biocidal inactivation treatments leads to possibilities to attack spores at different sites simultaneously, such as the spore's DNA, cytoplasm or spore coat. Such a multilevel approach could lead to a combination of treatments that are complementary and also provide more effective treatments against a wider range of microorganisms. Industrial relevance The combination of Nd:YAG laser and UV irradiation treatment of B. cereus spores was found to be an effective treatment. Experiments were done on agar, stainless steel and in distilled water. The combination of decontamination processes or agents provides opportunities for increased decontamination efficacy against a broader range of substrates and microorganisms.