We attempted to clarify in detail the conditions of disinfection using electrolyzed strongly acidic water (ESW) against Mycobacteria, and the recovery of the disinfection potential of inactivated ESW by re-electrolysis. We mixed ESW containing 10, 20, and 30 ppm free chlorine with M. bovis cells (10 5 -10 8 CFU/mL) for 0-7 min. The disinfection potential of ESW positively correlated with free chlorine concentration, and negatively correlated with the initial density of bacterial cells. To clarify the recovery of the disinfection potential of inactivated ESW by re-electrolysis, we mixed ESW containing 10 ppm free chlorine with M. bovis cells (10 7 CFU/mL) for 1 min. The number of viable cells decreased to 1/10 3 , but the cells were still detected. After re-electrolysis for 7 min, viable cells were not detected. Moreover, we confirmed by reusing the re-electrolyzed water against M. bovis cells that it regained its disinfection potential. These findings indicate that ESW once inactivated during disinfection can be re-activated by re-electrolysis. In conclusion, we were able to clarify in detail the conditions of ESW against Mycobacteria, and found the recovery of the disinfection potential of inactivated ESW by re-electrolysis.Electrolyzed strongly acidic water (ESW) has been widely used for infection control in clinical settings (7, 9) because it has been proved to be effective for the disinfection of disinfectant-resistant bacteria, such as Mycobacteria (15), Pseudomonas aeruginosa (6), and spore-forming bacteria (3, 4, 6), and viruses causing blood-borne infections such as hepatitis B virus (8, 11) and HIV (5). ESW is also expected to be used against pathogens of emerging diseases, such as the avian influenza virus H5N1 (12). Free chlorine (residual chlorine), which is the active component of ESW for disinfection, is easily inactivated by organic molecules. This feature of ESW is advantageous in terms of reducing environmental load and safety for patients and healthcare workers; however, it may be a disadvantage due to its unstable disinfective effect. Considering the effectiveness and the stability of the disinfective effect of ESW, high free chlorine concentrations are better for infection control; however, ESW with high free chlorine concentrations is corrosive against metals. There are no data available regarding the relationship between the free chlorine concentration of ESW and the number of bacterial

Glutaraldehyde, a germicide for reprocessing endoscopes that is important for hygiene in the clinic, might be hazardous to humans. Electrolyzed acid water (EAW) has a broad anti-microbial spectrum and safety profile and might be a glutaraldehyde alternative. We sought to assess EAW disinfection of flexible endoscopes in clinical otorhinolaryngological settings and its in vitro inactivation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and bacteria commonly isolated in otorhinolaryngology. Ninety endoscopes were tested for bacterial contamination before and after endoscope disinfection with EAW. The species and strains of bacteria were studied. The in vitro inactivation of bacteria and SARS-CoV-2 by EAW was investigated to determine the efficacy of endoscope disinfection. More than 20 colony-forming units of bacteria at one or more sampling sites were detected in 75/90 microbiological cultures of samples from clinically used endoscopes (83.3%). The most common genus detected was Staphylococcus followed by Cutibacterium and Corynebacterium at all sites including the ears, noses, and throats. In the in vitro study, more than 107 CFU/mL of all bacterial species examined were reduced to below the detection limit (<10 CFU/mL) within 30 s after contact with EAW. When SARS-CoV-2 was treated with a 99-fold volume of EAW, the initial viral titer (> 105 PFU) was decreased to less than 5 PFU. Effective inactivation of SARS-CoV-2 was also observed with a 19:1 ratio of EAW to the virus. EAW effectively reprocessed flexible endoscopes contributing to infection control in medical institutions in the era of the coronavirus disease 2019 pandemic.

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