Effect of Nalidixic Acid Adaptation on Shiga Toxin and Non-pathogenic Escherichia coli to Radio Frequency Heating

Introduction: O157:H7 and non-O157 Shiga Toxin Escherichia coli (STEC) are recognized as major foodborne pathogens resulting in serious health issues such as HUS and kidney failure. Radio Frequency (RF) uses electromagnetic energy to heat foods with rapid heat distribution and relatively lower energy consumption. RF heating can be used to reduce high levels of microbial contamination in foods such as beef. Efficacy of new interventions to control STEC that use antibiotic resistant pathogens must be validated for similar resistance to interventions as parent strains.

Purpose: To validate the use of Nalidixic acid- (NA-) adapted strains of three STEC and non-pathogenic E. coli in studies of the efficacy of RF heating using a model solution.

Methods: A total of 15 parent strains and 15 NA strains of E. coli O157:H7, O26:H11 and O111 along with 4 non-pathogenic surrogates were used on this study.  Cultures of each strain were centrifuged and combined in equal proportions to prepare cocktails of respective serotype.  Phosphate buffered saline solutions were inoculated with cocktail and heated using an RF oven to three different endpoint temperatures (55°C, 60°C and 65°C) to be appropriately enumerated.

Results: Results showed log reduction between 0.5-1.0 for strains treated at 55°C, between 1.0-1.5 for strains treated at 60°C and above 5.0 for strains treated at 65°C for all serotype cocktails.  No significant differences were observed between parent and NA strains for each temperature.  

Significance: Results revealed that there was a significant difference in log reduction comparing 55°C and 60°C to 65°C.  There was not a significant difference between parent and NA-adapted strains which suggests that NA-adapted strains might be a good choice to substitute the parent strains for RF experiments in beef.