Abstract
Bacillus subtilis spore inactivation mechanisms under low energy electron beam (LEEB) and high energy electron beam (HEEB) treatment were investigated using seven mutants lacking specific DNA repair mechanisms. The results showed that most of the DNA repair-deficient mutants, including ΔrecA, ΔKu ΔligD, Δexo Δnfo, ΔuvrAB and ΔsbcDC, had reduced resistances towards electron beam (EB) treatments at all investigated energy levels (80 keV, 200 keV and 10 MeV) compared to their wild type. This result suggested DNA damage was induced during EB treatments. The mutant lacking recA showed the lowest resistance, followed by the mutant lacking Ku and ligD. These findings indicated that recA, Ku and ligD and their associated DNA repair mechanisms, namely, homologous recombination and non-homologous end joining, play important roles in spore survival under EB treatment. Furthermore, exoA, nfo, uvrAB, splB, polY1 and polY2, which are involved in nucleotide damage repair/removal, showed different levels of effects on spore resistance under EB treatment. Finally, the results suggested that HEEB and LEEB inactivate B. subtilis spores through similar mechanisms. This research will provide a better understanding of how EB technologies inactivate B. subtilis spores and will contribute to the application of these technologies as a non-thermal, gentle spore control approach.
Highlights
The inactivation curves of spores treated under high energy electron beam (HEEB) treatment (10 MeV, 1.55–7.61 kGy) were log10 linear with generally higher R2 values compared to low energy electron beam (LEEB) treatment
This study investigated spore inactivation mechanisms during LEEB and HEEB treatment by evaluating the D-values of B. subtilis wild type and seven mutants lacking relevant DNA repair mechanisms
The results revealed that DNA damage is one of the causes responsible for spore inactivation by both LEEB and HEEB treatments
Summary
Spore-forming bacteria of the genera Bacillus and Clostridia spp. are major sources of food spoilage and can cause food-borne diseases (Andersson et al, 1995; Mallozzi et al, 2010). They form resistant bacterial spores, which are the main targets of sterilization. Research has shown that EB is more gentle and preserves the quality of food products better compared to other ionizing irradiation technologies, e.g., gamma irradiation, and can effectively inactivate bacteria (De Lara et al, 2002; Fan et al, 2017; Fiester et al, 2012; Gryczka et al, 2018; Zhang et al, 2018). For HEEB, the D-values at 10 MeV ranged from 1.5 to 3.8 kGy for B. subtilis and Bacillus cereus spores (De Lara et al, 2002)
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