Escherichia coli disinfection by electrohydraulic discharges

We study the survival of single-strain Escherichia coli colonies in aqueous media exposed to 5.5 kV, 90 kA electrohydraulic discharges (EHD). The prob ability of survival (P-n) of a 4 x 10(7) cfu mL(-1) E. coli population afte r n consecutive EHDs follows a logit distribution: ln(Pd 100 - P-n) = 1.329 - 1.579 In n with r(2) = 0.993 that corresponds to lethal doses of LD50 = 2.2 and LD90 = 10.5 EHDs. Considering that the reactor is thoroughly mixed during each discharge and that LD50 = 0.9 values are nearly independent of E coli concentrations in the range of 2 x 10(3) less than or equal to E col i/cfu mL(-1) less than or equal to 3 x 10(6), we ascribe the nonexponential P-n decay of single-strain E coli colonies to a shielding phenomenon where inactive cells protect the successively smaller numbers of viable cells in the EHD. The qualitatively similar concentration dependence observed for s urvival under 254 nm of radiation, in contrast with the lower resistance of denser colonies to 20 kHz power ultrasound and the delayed onset of extrac ellular beta -D-galactosidase activity in bacterial populations already dec imated by ENS, support the view that UV radiation is the dominant disinfect ion agent generated by electrohydraulic discharges.