Effect of u.v. light irradiation, starvation and heat on Escherichia colibetabeta-D-galactosidase activity and other potential viability parameters

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Abstract
The effect of u.v. light irradiation and two other types of stress (heat and starvation) on cellular functions of Escherichia coli have been studied. The severe reduction of the culturable cell number (cfu) and the direct viable count (DVC) after exposure to moderate u.v. light doses (48 mWs cm–2), was not reflected by the dehydrogenase activity (5-cyano-2,3-ditolyl tetrazolium chloride (CTC)-positive cells), the membrane integrity (SYTOX Green-negative cells), the membrane potential (bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4[3]) (OXONOL)-negative cells), and the β- d-galactosidase activity. All parameters were affected by high u.v. light doses. Cellular activities (CTC, SYTOX, OXONOL, β- d-galactosidase activity) were intact in non-culturable cells with presumably severe damage to DNA, and the activities seemed not to be appropriate for detection of viable E. coli after u.v. light irradiation. Heating for 20–30 min at 63 °C was required to cause a severe loss of the β- d-galactosidase activity and the numbers of CTC- positive, SYTOX Green-negative or OXONOL-negative cells. A large portion (≥38%) of pre- irradiated (190 mWs cm–2) cells maintained their ability to reduce CTC and exclude SYTOX Green and OXONOL after 51 d of starvation (dark, 7 °C) in phosphate-buffered saline.