UV-C light is a nonthermal treatment widely used for disinfecting water and surfaces (Koutchma, Popović, Ros-Polski, & Popielarz, 2016). It is a physical treatment capable of affecting the DNA of the microbial cells, causing crosslinking between neighboring pyrimidine bases, which affects the replication and transcription process causing cell death (Ochoa-Velasco et al., 2020). Nowadays, it is used as a pasteurization method for the treatment of food commodities such as beverages, juices, milk, and liquid eggs (Beristaín-Bauza et al., 2018; Crook, Rossitto, Parko, Koutchma, & Cullor, 2015; De Souza et al., 2015; García Carrillo, Ferrario, & Guerrero, 2017; Gunter-Ward et al., 2017; Koutchma et al., 2016), and fresh vegetables for reducing external microorganisms as well as an abiotic treatment for increasing the biosynthesis of secondary metabolites (phenolic acids, flavonoids, carotenoids, terpenoids, etc.) currently demanded by the consumers (Avalos-Llano, Molina, & Sgroppo, 2020; Esua, Chin, Yusof, & Sukor, 2019; Martínez-Zamora, Castillejo, & Artés-Hernández, 2021; Pérez-Ambrocio et al., 2018). On the other hand, in areas related to environmental protection, UV light is used to remove organic constituents from industrial wastewater through an advanced oxidation process (Rekhate & Srivastava, 2020). In this process, UV light photocatalysts many organic compounds through the generation of radicals such as HO● and O2●−, which are effective in destroying organic compounds because they are reactive electrophiles that rapidly react with nearly electron-rich organic compounds by radical addition, hydrogen abstraction, and electron transfer (Stasinakis, 2008; Sujatha, Shanthakumar, & Chiampo, 2020).

Phenols are compounds with a single or more aromatic rings coupled to a single or more hydroxyl groups (Alara, Abdurahman, & Ukaegbu, 2021). They are widely distributed in natural products, ranging from phenolic acids to complex compounds such as tannins (Luna-Guevara, Luna-Guevara, Hernández-Carranza, Ruíz-Espinosa, & Ochoa-Velasco, 2019). In plants, phenols act as part of their defense system against biotic and abiotic stress factors, being responsible for some sensory properties of the plant food products (Alara et al., 2021). Phenolic compounds are interesting bioactive compounds because they can act as an antioxidant, antimicrobial, anti-inflammatory, etc., gaining attention due to their several applications in the food, biotechnology, and pharmaceutical applications (Albuquerque, Heleno, Oliveira, Barros, & Ferreira, 2021). In this regard, an interesting property of phenolic compounds is their antimicrobial activity, primarily associated with the presence of hydroxyl groups in their structure, having the ability to interact with membranes and other active sites of bacterial cells (Lima et al., 2019).

GA (3,4,5-trihydroxy benzoic acid) is a phenolic compound naturally found in various fruits and vegetables. Several phenolic compounds like GA and other phenolic acids (ρ-coumaric, caffeic, ferulic and sinapic acids), as well as flavonoids (coumarin, naringenin, quercetin, rutin, catechin, etc.) have been shown to possess antimicrobial activity. Despite that some phenolic compounds present higher antimicrobial activity than GA (Díaz-Gómez, López-Solís, Obreque-Slier, & Toledo-Araya, 2013; Maddox, Laur, & Tian, 2010; Pernin, Guillier, & Dubois-Brissonnet, 2019; Wang, de Oliveira, Alborzi, Bastarrachea, & Tikekar, 2017), the use of GA is preferred due to its “safe” nature, food occurrence, high availability, low cost, high-water solubility, and some other beneficial health properties (Bai et al., 2021; Chuang & Hsieh, 2023; Sorrentino et al., 2018). Interestingly, the antimicrobial activity of GA can increase with the application of UV light due to the generation of reactive oxygen species (ROS) and photo-oxidized compounds. In this aspect, Wang et al. (2017) and Wang, Leong, Elias, and Tikekar (2019) reported that UV-A and UV-C lights increased the antimicrobial activity of GA against E. coli O157:H7. The authors pointed out that the UV-A and UV-C light irradiation of GA (15 mM) for 30 min increased the antimicrobial capacity of GA, attaining a reduction of 4.4 and 3.2-log CFU/mL, respectively.

Although the current results encourage applying UV light to aid in the antimicrobial effect of GA, more research is required to understand the mechanisms of bacterial inactivation. In this regard, among the main factors that can affect the antimicrobial activity in IGA treatments are the irradiation time and the GA concentration. In addition, to date, the reactivation process of E. coli after IGA treatment is still unexplored. Therefore, this study aimed to evaluate the effect of GA concentration, irradiation time, and cell concentration on the microbial inactivation of E. coli, considering both the IGA treatments alone and the sequential combination of the IGA and UV-C irradiation treatments. To fully achieve this purpose, the following experimental tasks were performed: a) assess the effect of contact time and microbial load on the antimicrobial activity of GA against E. coli, b) evaluate the application of GA (30 min) plus UV-C light treatment on the microbial reduction of E. coli, c) assess the effect of irradiation time and GA concentration on the antimicrobial activity of GA, and d) evaluate the reactivation process of IGA-treated E. coli after 24 h of storage at 22 and 37 °C.