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
beta-D-galactosidase activity. All parameters were affected by high u.v. light doses. Cellular activities (CTC, SYTOX, OXONOL,
beta-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 degrees C was required to cause a severe loss of the
beta-D-galactosidase activity and the numbers of CTC-positive,
SYTOX Green-negative or OXONOL-negative cells. A large portion (> or = 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 degrees C) in
phosphate-buffered saline.