Synergistic antimicrobial activity based on the combined use of a gemini-quaternary ammonium compound and ultraviolet-A light

• Gemini-quaternary ammonium salt and UV-A light were combined to disinfect bacteria.
• The combined disinfection decreased the viability of bacteria rapidly.
• The decrease in viability depended on doses of the biocide and the UV-A fluence.
• The combination caused overproduction of reactive oxygen species in cells.
• The combination facilitated lipid peroxidation and DNA oxidation.

This study examined the utility of synergistic disinfection employing a gemini-quaternary ammonium salt (a gemini-QUAT, namely 3,3′-(2,7-dioxaoctane)bis(1-decylpyridinium bromide)), as an organic biocide in combination with irradiation by an ultraviolet-A (UV-A) light-emitting diode (LED) with a peak wavelength of 365 nm. The combined system represents a novel disinfection method utilizing facilitated in situ oxidation depending on overproduction of reactive oxygen species (ROS) triggered by the initial action of the gemini-QUAT on the bacterial membrane. We demonstrate that this combination decreased the viability of pathogenic bacteria in a significant and rapid manner, and depended on doses of the gemini-QUAT and the fluence: the viability of Escherichia coli was reduced by greater than 5.0-logs by the combination procedure, but the decrease in viability was only 2.3-logs for exposure to UV at the same fluence dose in the absence of the gemini-QUAT. Adding catalase as a radical scavenger decreased bacterial inactivation by the combined disinfection procedure. Flow cytometric analysis indicated superoxide and hydrogen peroxide overproduction within cells treated with the combined disinfection procedure. The excessive superoxide, detected only in the combined system, appeared to be generated by the action of the gemini-QUAT at the bacterial membrane, leading to excessive and rapid generation of ROS in the system. Our data strongly suggested that this ROS promoted bacterial membrane peroxidation during initial treatment by the combination method, resulting in increased oxidative modification of DNA. These oxidative reactions may play an important role in the efficacy of this disinfection procedure.