Hydrogen Peroxide Augments the Disinfection Efficacy of 280 nm Ultraviolet LEDs against Antibiotic-Resistant Uropathogenic <i>Escherichia coli</i> Otherwise Tolerant to Germicidal Irradiation

The 280 nm ultraviolet light-emitting diode (UV LED) has emerged as a promising water disinfection technology and has been applied from point-of-use devices to municipal-scale systems. Accumulating evidence suggests that small numbers of antimicrobial-resistant bacteria can survive UV treatment; however, little research has been done to evaluate 280 nm UV alone or as part of advanced oxidation processes against these organisms. Here, we investigated 280 nm UV LED inactivation of a high-risk uropathogenic <i>Escherichia coli</i> (<i>E. coli</i>) strain belonging to clade A of the globally distributed, antimicrobial-resistant ST131 lineage known to persist in treated wastewater (ST131-A). We measured disinfection efficacy with and without hydrogen peroxide (H₂O₂) and assessed the reactivation of the treated bacteria in the light and dark. Our bench-scale modeling revealed that ST131-A displays UV tolerance, requiring 2.8 times more UV dose (35 mJ/cm²) for 6-log₁₀ inactivation compared to the reference strain <i>E. coli</i> ATCC 25922 (12.5 mJ/cm²). Furthermore, we observed ∼1% photoreactivation of <i>E. coli</i> ST131-A even after UV doses of 90 mJ/cm², which is significantly greater than the operational minimums reported for the world's first 280 nm UV wastewater treatment plant. Analysis of a genome-edited bacterial model (<i>Pseudomonas aeruginosa</i>) suggests that bacterial reactivation following 280 nm UV treatment is linked to photolyase activity. The addition of 10-200 mg/L H₂O₂ increased the inactivation of <i>E. coli</i> ST131-A 6,000-300,000-fold compared to UV or H₂O₂ treatments alone, respectively. Chemical and microbiological analyses revealed that the mechanism of increased inactivation by combined treatments relied on UV photolysis of H₂O₂ to generate hydroxyl radicals and residual H₂O₂ activity that inhibited bacterial reactivation after treatment. Our findings suggest that some <i>E. coli</i> ST131 strains possess high UV tolerance, and that UV/H₂O₂ treatment may effectively inactivate these multidrug- and UV-tolerant uropathogenic strains and thwart their reactivation in treated water.