Purpose: In this study, factors involved in spore resistance to cold plasma were investigated.
Methods: Bacillus subtilis spores and isogenic mutant strains were treated using a dielectric barrier discharge plasma system. The strains PS578 (α- β-), which lacks the genes encoding the two major α/β-type small acid soluble proteins (SASP); FB122 (sleB spo VF), which lacks dipicolinic acid (DPA); PS3328 (cotE), which lacks the outer coat, and the wild type PS832 were treated up to five minutes in a static atmosphere using different process gases (air, N2, O2). Plasma was characterized using optical emission spectroscopy and by the quantification of ozone.
Results: Air-plasma showed high emission intensities in the UV-A and UV-B range. N2-plasma emitted, mainly, UV-C photons. O2-plasma generated a high amount of reactive oxygen species (ROS) and up to 22,000 ppm ozone. The strains PS3328 and FB122 were sensitive to the O2-plasma, with an inactivation of 4.1 and 3.8 log10 after 5 min; the other strains showed a reduction of 2.7 log10. Strain PS578 was sensitive to the N2-plasma treatment, with a reduction of 4.8 log10 after 0.5 min. The other strains were inactivated by 3.1 (PS832), 2.8 (PS3328) and 1.7 (FB122) log10. When air was the process gas, strain PS578 showed the highest inactivation with 4.8 log10 after 5min; PS832 and PS3328 were inactivated by 3.1 and FB122 by 2.1 log10, respectively .
Significance: The results indicated the different factors involved of B. subtilis spore resistance to cold plasma. The α/β-type SASP play a significant role in spore resistance to the emitted UV-C photons and, furthermore, protect the outer coat and the spore DPA against generated ROS.