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 probability of survival (P_n) of a 4 × 10^7 cfu mL^(-1) E. coli population after n consecutive EHDs follows a logit distribution:  ln(P_n/100 − P_n) = 1.329 − 1.579 ln n with r^2 = 0.993 that corresponds to lethal doses of LD_(50) = 2.2 and LD_(90) = 10.5 EHDs. Considering that the reactor is thoroughly mixed during each discharge and that LD_(50) = 0.9 values are nearly independent of E. coli concentrations in the range of 2 × 10^3 ≤ E. coli/cfu mL^(-1) ≤ 3 × 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 survival under 254 nm of radiation, in contrast with the lower resistance of denser colonies to 20 kHz power ultrasound and the delayed onset of extracellular β-d-galactosidase activity in bacterial populations already decimated by EHDs, support the view that UV radiation is the dominant disinfection agent generated by electrohydraulic discharges.