## 1 Introduction  
_Vibrio parahaemolyticus_ is a Gram-negative marine bacterial species that can cause acute gastroenteritis manifested by diarrhea, headache, vomiting, nausea, and abdominal pain; this is typically observed in individuals who have consumed raw or undercooked seafood [1; 2; 3; 4]. _V. parahaemolyticus_-induced gastroenteritis is mediated by the type III secretion system and by enterotoxins, including thermostable direct hemolysis (TDH) and TDH-related hemolysis (TRH) [5; 6; 7]. Food poisoning associated with _V. parahaemolyticus_ is a frequent occurrence worldwide [8; 9]. Chlorination and ozonation have high efficiency against bacteria in general. However, some health problems have been observed. For instance, residual chlorine in drinking water can cause the formation of potentially carcinogenic halogenated by-products [10]. Likewise, ozonation can lead to the formation of high concentrations of undesired by-products, including bromates, which are also potential human carcinogens [11]. Other well-known disinfection methods include sunlight and ultraviolet (UV) irradiation; these modalities produce by-products, but those reported so far are below the level of health concerns [12]. UV rays are classified by wavelength into UVA (320-400 nm), UVB (280-320 nm), and UVC (<280 nm). Solar disinfection is an effective and inexpensive method of water treatment due to the fact that UVA and partial UVB in sunlight pass the ozonosphere and reach the surface of the Earth; this is not the case for UVC. UVC-based disinfection systems that utilize low-pressure mercury lamps are used widely as an effective sterilization method for both drinking and wastewater [13]. Because deoxyribonucleic acid (DNA) has a maximum absorption at approximately the same wavelength as UVC, irradiation can introduce photoproducts, including cyclo-butane pyrimidine dimers (CPDs) into the genomic DNA of target bacteria [14; 15]. Some bacterial species have systems that repair thymine dimmers, including photolyases and the SOS response; these properties impart tolerance to the effects of UVC on bacterial DNA [16; 17]. We originally developed a UVA irradiation disinfection system based on a light-emitting diode (UVA-LED) [14]. This system was capable of disinfection, and was specifically useful for the elimination of enteropathogenic bacteria, including _Escherichia coli_ or _Vibrio parahaemolyticus_[18; 19]. Our findings and those of other groups revealed that UVA irradiation induces cellular membrane damage and indirectly results in delayed growth by increasing intracellular levels of reactive oxygen species (ROS), including superoxide anion radicals (O\({}_{2}\)\({}^{\bullet}\)), hydroxyl radicals (OH\({}^{\bullet}\)), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), and singlet oxygen (\({}^{1}\)O\({}_{2}\)) [14; 18; 19]. Furthermore, we recently reported that bacterial systems that were effective for repairing DNA damage secondary to UVC irradiation were not effective against damage elicited by UVA-LED irradiation [18]. Unfortunately, we found that UVA-LED irradiation had lower bactericidal efficiency than UVC did against _V. parahaemolyticus_[18; 19]. As such, we performed the current study in order to learn more about bacterial sensitivity to UVA-LED. We were particularly interested in exploring factors underlying the UVA sensitivity of _V. parahaemolyticus_. As such, the aim of this study was to identify genes encoded by _V. parahaemolyticus_ that are associated with UVA sensitivity. Toward this end, we constructed a _V. parahaemolyticus_ mutant library using a transposon mutagenesis method; we then identified mutant strains with increased sensitivity to UVA-LED irradiation. In this study, we isolated and characterized six bacterial strains that displayed increased sensitivity to UVA-LED irradiation; among these were strains with mutations in genes encoding the F\({}_{0}\)F\({}_{1}\)-type ATP synthase subunits (_atp_) as well as those involved in the biosynthesis of ubiquinone (_ubi_) and the general secretion pathway (_gsp_) genes.