Purpose: This study was designed to validate the use of nalidixic acid-resistant (NalR) strains of various STEC for electrolyzed oxidizing (EO) water efficacy study.
Methods: The resistance of total sixty-two strains (parent strains {NalS} and NalR) of E. coli O157:H7 and six major serotypes of non-O157 STEC was tested against EO water using minimum inhibitory concentration (MIC) and inoculated beef trims. MIC was conducted for 15 s testing period with free chlorine concentration of 3.00, 2.50, 2.00, 1.50, 1.00, 0.50 and 0.25 mg/l. Beef pieces (5 cm cube, 25 -35% fat) were inoculated with same serogroup strains cocktail and treated with EO water (70mg/l available chlorine, pH 2.5, ORP 1175mV) for 5 min. At the end of each treatment period, beef pieces were promptly placed in 2X neutralizing buffer and bacterial counts were determined by plating appropriate dilutions on plate count agar and MacConkey agar plates (supplemented with or without 50 mg/l nalidixic acid).
Results: The MIC of individual strains ranged from 0.25 to 1.50 mg/l free chlorine of EO water. Resistance pattern of cocktails was determined as E. coli O157 ≥ O103 ≥ O26 ≥ 0111 ≥ O121 ≥ 045 > O145. When treated on beef trim similar pattern of resistance was observed with pathogen load reduction ranging from 1.5 to 2.8 CFU/cm2. No significant differences in sensitivity towards EO water treatment were observed between NalS and their NalR derivatives in either method.
Significance: No significant difference in resistance of NalR and NalS were found which validates the use of NalR strains in EO water efficacy study. In addition, the EO water treatment that reduces E. coli O157:H7 could equally or more effectively reduce non-O157 STEC from beef trim.