Assessing the Antimicrobial Mode of Action of Novel Non-Thermal Processes through the Use of Escherichia coli Mutants

The efficiency of food processes have been associated with different stress environments such as thermal shock, oxidative stress, and the presence of antimicrobial compounds. It is known that bacteria have evolved different defense and repair mechanisms to defend themselves against the stresses generated by these technologies. This work reviews a number of stress environments and the microbiological responses in relation to cold atmospheric plasma and ozone treatments. The regulatory mechanisms of the bacterial responses against reactive oxygen species and thermal stress compounds are showcased. For that purpose, ΔsoxR, ΔsoxS, ΔoxyR, ΔrpoS, ΔdnaK mutants whose deleted genes are mainly transcriptional regulators (soxR, soxS, oxyR and rpoS) capable of responding to, amongst others, hydrogen peroxide and superoxide radicals, were studied as representative markers in microbial stress environments. The macroscopic responses of these mutants where quantified, during treatments, by cold atmospheric plasma (70 kV, 50 Hz for treatments of 15, 30, 60, 90 s) and gaseuous ozone (6 μg/ml for 240 s), in order to interpret the antimicrobial mechanisms of action. The recovery growth rates of the knockout mutants resulted in different responses. For example, ΔsoxS was not recovered following a 90 s plasma treatment indicating the presence of reactive oxygen species, such as nitric oxide and superoxide-generating agents, during this treatment. Overall, functional genomics via mutant libraries can permit better interpretation of the adaptions of the microbial physiology.