Research noteInfluence of nalidixic acid adaptation on sensitivity of various Shiga toxin-producing Escherichia coli to EO water treatment

This study was designed to validate the use of nalidixic acid-adapted strains of various STEC for electrolyzed oxidizing (EO) water efficacy testing. The resistance of total 48, parent (NalS) and adapted (NalA) strains of Escherichia coli O157:H7 and six major serotypes of non-O157, STEC were tested against EO water using minimum inhibitory concentration (MIC) and inoculated beef trims. MIC was conducted for 15 s testing period with free chlorine concentrations of 3.00, 2.50, 2.00, 1.50, 1.00, 0.50 and 0.25 mg/L. While, beef trims (5 cm cubes) were inoculated with a cocktail of the same serogroup strains and treated with EO water (50 mg/L available chlorine) for 1 min. The MIC values of individual strains ranged from 0.41 to 1.66 mg/L free chlorine of EO water. When treated on beef trim, pathogen load reductions ranging from 0.44 to 1.54 log CFU/cm2 were observed. No significant differences in sensitivity towards EO water treatment were observed between NalS and their NalA derivatives in either study which validates the use of NalA strains in EO water efficacy study. In addition, the EO water treatment that reduced E. coli O157:H7 was equally or more effective in reducing non-O157 STEC on beef trim.