UV inactivation, liquid-holding recovery, and photoreactivation of Escherichia coli O157 and other pathogenic Escherichia coli strains in water

Drinking water, water used in food production and for irrigation, water for fish farming, waste water, surface water, and recreational water have been recently recognized as a vector for the transmission of pathogenic Escheri chia coli, especially serotype O157:H7. We investigated the UV (253.7 nm) i nactivation behavior and the capability of dark repair (liquid-holding reco very) and photoreactivation of seven pathogenic (including three enterohemo rrhagic E. coli) strains and one nonpathogenic strain of E. coli (ATCC 1122 9) with respect to the use of UV light for water disinfection purposes. Bec ause most bacteria and yeast are known to be able to repair UV damage in th eir nucleic acids, repair mechanisms have to be considered to ensure safe w ater disinfection. We found a wide divergence in the UV susceptibility with in the strains tested. A 6-log reduction of bacteria that fulfills the requ irement for safe water disinfection was reached for the very most susceptib le strain O157:H7 (CCUG 29199) at a UV fluence of 12 J/m(2), whereas for th e most resistant strain, O25:K98:NM, a UV fluence of about 125 J/m(2) was n eeded. Except for one strain (O50:H7) liquid-holding recovery did not play an important role in recovery after UV irradiation. By contrast, ail strain s, particularly strains O25:K98:NM, O78:K80:H12, and O157:H7 (CCUG 29193), demonstrated photorepair ability. For a 6-log reduction of these strains, a UV fluence (253.7 nm) up to 300 J/m(2) is required. The results reveal tha t the minimum fluence of 400 J/m(2) demanded in the Austrian standard for w ater disinfection is sufficient to inactivate pathogenic E, coli. A fluence of 160 J/m(2) (recommendation in Norway) or 250 J/m(2) (recommendation in Switzerland) cannot be regarded as safe in that respect.