Ultraviolet-C (UV-C) irradiation is a promising approach for inactivating viruses and microorganisms. Germicidal mercury lamps emitting at 254 nm and krypton chloride (KrCl) excimer lamps emitting at 222 nm both possess sterilization properties, but the KrCl lamp is considered less harmful to humans. UV-C at 222 and 254 nm induces cyclobutene pyrimidine dimers (CPDs) in Escherichia coli (E. coli). Although E. coli irradiated at 254 nm can undergo photoreactivation, cells exposed to 222 nm cannot, as CPDs are not effectively repaired. This study aimed to investigate the molecular mechanisms underlying bacterial inactivation by UV-C irradiation at 222 and 254 nm. We measured the absorbance of key bacterial components–peptidoglycan (PG), membrane proteins, phospholipids, and DNA–in Gram-negative and Gram-positive bacteria and confirmed the photodecomposition. At 222 nm, a substantial fraction of the light was absorbed by the membrane proteins, and the cleavage of peptide bonds resulted in structural damage and leakage of cellular contents, leading to cell death. In Gram-positive bacteria, which contain thick PG layers, part of the UV-C was absorbed by PG. Irradiation with 222-nm light of PG and proteins resulted in peptide bond cleavage, liberating amino acids. Degradation of the bacterial cell envelope increased the permeability and efflux of intracellular substances, leading to membrane rupture and, ultimately, bacterial cell death. These findings demonstrate that 222-nm irradiation exerts bactericidal effects through distinct mechanisms in Gram-negative and Gram-positive bacteria.
Synopsis
The Gram-negative E. coli strain NBRC 106373 and the Gram-positive S. aureus strain NBRC 12732 undergo DNA mutations when irradiated at 222 nm. Simultaneously, they are killed by the leakage of cellular content. This is because the strong light absorption of peptidoglycan and membrane proteins damages the cell structure.
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