Mechanism of the Synergistic Inactivation of Escherichia coli by UV-C Light at Mild Temperatures

Abstract

UV light only penetrates liquid food surfaces to a very short depth, thereby limiting its industrial application in food pasteurization. One promising alternative is the combination of UV light with mild heat (UV-H), which has been demonstrated to produce a synergistic bactericidal effect. The aim of this article is to elucidate the mechanism of synergistic cellular inactivation resulting from the simultaneous application of UV light and heat. The lethality of UV-H treatments remained constant below ϳ45°C, while lethality increased exponentially as the temperature increased. The percentage of synergism reached a maximum (40.3%) at 55°C. Neither the flow regimen nor changes in the dose delivered by UV lamps contributed to the observed synergism. UV-H inactivation curves of the parental Escherichia coli strain obtained in a caffeic acid selective recovery medium followed a similar profile to those obtained with uvrA mutant cells in a nonselective medium. Thermal fluidification of membranes and synergistic lethal effects started around 40 to 45°C. Chemical membrane fluidification with benzyl alcohol decreased the UV resistance of the parental strain but not that of the uvrA mutant. These results suggest that the synergistic lethal effect of UV-H treatments is due to the inhibition of DNA excision repair resulting from the membrane fluidification caused by simultaneous heating. U V-C light is an emerging disinfection technology for water and, more recently, for liquid foods due to its multiple advantages (1, 2). UV-C (220 to 300 nm) has a germicidal effect for most types of microorganisms because it produces photochemical modifications of nucleic acids' pyrimidine bases. The major UVinduced DNA lesion is cyclobutane pyrimidine dimers (CPDs), while (6-4) photoproducts (6-4PPs) are also formed on about 25% of CPDs (3). These lesions prevent the proper replication and transcription of DNA, resulting in mutagenesis and, ultimately, cell death (4). The magnitude of the lethal effect depends on the radiation dose and on the cells' ability to repair damage.Microorganisms have adopted various enzymatic DNA repair pathways to restore DNA molecules from replication errors and the action of both endogenous and exogenous DNA-damaging agents. The DNA repair pathways involved in damage repair prior to replication include photorepair, base excision repair (BER), and nucleotide excision repair (NER) (3, 4). Under extensive DNA damage, repair mechanisms controlled by the SOS regulon, such as RecA-mediated excision repair (RAMER), translesion synthesis (TLS), and homologous recombination (HR) repair, are induced (4, 5). Overall, the lethality of UV light could be improved by impairing bacterial DNA repair mechanisms.The ability of UV light to be used for liquid food hygienization has been widely demonstrated (2, 6). In fact, UV-based technologies have been approved as alternative treatments to thermal pasteurization of fresh juice products (7). However, the implementation of UV processing in the food industry is still limited due to ...

Inactivation of spoilage yeasts in apple juice by UV–C light and in combination with mild heat

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Fundamental Characteristics of Deep-UV Light-Emitting Diodes and Their Application To Control Foodborne Pathogens

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Impact of temperature on the survival and the biocontrol efficacy of Lysobacter capsici AZ78 against Phytophthora infestans

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