Resistance of Parent and Nalidixic Acid Adapted (NA) Escherichia coli O157:H7 and Other Shiga Toxin-producing Non-O157 Escherichia coli to Ultraviolet Treatment

Introduction: Shiga toxin-producing Escherichia coli (STEC) known to cause severe life threatening health conditions like HUS is linked to a wide variety of foods in general, but beef is a predominantly identified source of serious E. coli O157:H7 and non-O157 STEC outbreaks.  

Purpose: Current research aims at determining the D and Z of different STECs to UV at three different intensities. The research also aims at validating the usage of Nalidixic acid adapted (NA) strains to replace the parent strains for UV radiation treatments of beef.

Methods: A total of 20 parent strains and 20 NA strains of E. coli O157:H7, O26, O45, O103, O104, O111, O121 and O145 along with 4 nonpathogenic surrogates were used in this study (individual/cocktail). Overnight grown cultures were centrifuged and suspended in phosphate buffered saline and treated with UV at three different intensities (1.04, 1.92 and 3.02 mW/cm²/sec) and appropriately enumerated.

Results: The D-values were determined from the linear portion of the survival curves. The results show that among the five E. coli O157:H7 strains tested, strain 5 was the most resistant strain with a D-value of 86.8 sec at 1.04 mW/cm² and E. coli O104 was the most resistant non O157 serotype with D-value of 65.8 sec. All the NA strains were significantly less resistant to UV than the parent strains except for E. coli O157:H7 strain E932 which had a D-value of 64.6 sec.

Significance: Results revealed that the D-values of parent strains were significantly more resistant than the NA strains which suggest that NA strains might not be a good choice to substitute the parent strains for UV experiments and also high intensity short time UV treatments were more effective than low intensity long time treatments for the same dosage.