Roles of individual radicals generated by a submerged dielectric barrier discharge plasma reactor during Escherichia coli O157:H7 inactivation

Abstract

A submerged dielectric barrier discharge plasma reactor (underwater DBD) has been used on Escherichia coli O157:H7 (ATCC 35150). Plasma treatment was carried out using clean dry air gas to investigate the individual effects of the radicals produced by underwater DBD on an E. coli O157:H7 suspension (8.0 log CFU/ml). E. coli O157:H7 was reduced by 6.0 log CFU/ml for 2 min of underwater DBD plasma treatment. Optical Emission Spectra (OES) shows that OH and NO (α, β) radicals, generated by underwater DBD along with ozone gas. E. coli O157:H7 were reduced by 2.3 log CFU/ml for 10 min of underwater DBD plasma treatment with the terephthalic acid (TA) OH radical scavenger solution, which is significantly lower (3.7 log CFU/ml) than the result obtained without using the OH radical scavenger. A maximum of 1.5 ppm of ozone gas was produced during the discharge of underwater DBD, and the obtained reduction difference in E.coli O157:H7 in presence and in absence of ozone gas was 1.68 log CFU/ml. The remainder of the 0.62 log CFU/ml reduction might be due to the effect of the NO (α, β) radicals or due to the combined effect of all the radicals produced by underwater DBD. A small amount of hydrogen peroxide was also generated but does not play any role in E. coli O157:H7 inactivation.

A submerged dielectric barrier discharge plasma inactivation mechanism of biofilms produced by Escherichia coli O157:H7, Cronobacter sakazakii, and Staphylococcus aureus

“…In case of OH radical, it was observed that after 6–8 min of DBD operation, the intensity or concentration of 2-hydroxyterephthalic acid (HTA) or OH radical (1 mol of OH radical ≡ 1 mol of HTA) was decreased whereas the concentration of HTA (OH radical) was supposed to be either increased or remained constant. Decreasing phenomena of OH radical was described in detail with experimental evidence in the previous work10. Actually dissolved ozone, OH radical and H 2 O 2 concentration remains constant throughout the 90 min of plasma treatment after 4 min, 8 min and 6 min of plasma generation, respectively.…”

“…Among these radicals, the OH radicals play a major role in inactivation, with dissolved O 3 gas ranking next in importance. H 2 O 2 was found to play no role in E. coli O157:H7 inactivation in our study because the amount of hydrogen peroxide produced during our treatment time was very low (2.5 × 10 −6  M) and remains constant throughout the operation10. The inactivation mechanism of E. coli O157:H7 via the OH radical may proceed through some chemical or physical process; OH radicals are considered to enhance chemical reactions and also have a damaging effect on the unsaturated fatty acid side chains of lipid bilayers in various cell membranes6263.…”

“…Hence, more time is required to remove the biofilm from the SS coupon in comparison with the same population level of bacterial suspensions. In our previous work, we observed that the Escherichia coli O157:H7 suspension (~8.0 log) was completely inactivated within two and half minutes of underwater DBD plasma treatment10. However, once the EPS layers were disrupted and washed away from the SS coupon by water flow, then the next layer of biofilm was exposed to the plasma radicals and disrupted.…”

“…Underwater DBD plasma used in this experiment generates OH radicals, reactive nitrogen species (RNS), ozone gas (O 3 ) and hydrogen peroxide (H 2 O 2 )10. In our previous study, it was observed that after 6, 4 and 8 min of plasma treatment time the amount generated for OH radical was 1.81 × 10 −5  M, dissolved ozone gas was 1.5 ppm and H 2 O 2 was 2.5 × 10 −6  M, respectively.…”

“…Other chemicals, such as chlorine, formaldehyde and H 2 O 2 , are also not suitable for many applications, and they pose an environmental hazard as well as risks to human health9. In the case of UV and gamma ray irradiation, the energetic photons that are generated can seriously damage the structure of DNA and, most importantly, these photons are carcinogenic to the human body10. For all the reasons discussed above, biofilm control and removal demands the development of new strategies.…”