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Traditional culture methods may underestimate the tolerance of microorganisms to disinfectants because of the existence of viable but nonculturable or sublethally injured cells after disinfection.The selection of a strict method is crucial for the evaluation of disinfection performance.The actions of 2 typical disinfectants-ultraviolet (UV) and chlorine-on the fecal indicator Escherichia coli were investigated by the detection of culturability,membrane permeability,metabolic activity,deoxyribonucleic acid (DNA),and messenger ribonucleic acid (mRNA).During UV disinfection,the irreversible damages in the cell membrane and cellular adenosine triphosphate (ATP) were negligible at low UV doses (＜80 mJ/cm2).However,membrane permeability was damaged at low doses of chlorine (＜ 5 mg/L),leading to leakage of cellular ATP.Our study showed that a slight lesion in DNA was detected even at high doses of UV (400 mJ/cm2) and chlorine (＞5 mg/L) treatments.The decay of mRNA was more rapid than that of DNA.The degradation level of mRNA depended on the choice of target genes.After exposure to 50 mJ/cm2 UV dose or 5 mg/L chlorine for 30 min,the DNA damage repair function (RecA mRNA) was inhibited.The mRNA involved in the DNA damage repair function can be a potential indicator of bacterial viability.