Comparison of Shiga Toxin Subtypes and Chromosomal Insertion Sites in Escherichia coli O157 Isolated from Australia and the USA

Introduction: Escherichia coli O157 related disease in humans is primarily associated with the release of Shiga toxins encoded by genes associated with bacteriophages. Shiga toxin genes (stx) can be divided into two major types (stx₁ and stx₂) each of which comprises several subtypes. The diversity of stx subtypes carried by E. coli O157 has been shown to vary with respect to isolate source (cattle and human) and is believed to contribute to the variability in pathogenicity observed among genotypes.

Purpose: To determine the distribution of stx subtypes and stx-bacteriophage insertion sites in isolates derived from cattle and human sources from the USA (where large outbreaks have occurred) and Australia (very few, small outbreaks).

Methods: A total of 606 E. coli O157 isolates, including 284 from Australia (205 cattle and 79 human isolates) and 322 from the USA (143 cattle and 179 human isolates), were screened for stx subtypes (stx₁, stx₂ and stx_2c) and bilateral phage – chromosomal junctions of common stx-bacteriophage insertion sites (argW, sbcB, wrbA and yehV). 

Results: Australian isolates were more likely to carry both stx₁ and stx_2c (62%) than those from the USA (12%) where the combination of stx₁ and stx₂ predominated (55%). The phage insertion site argW was occupied by the majority (67%) of Australian isolates while only 21% of isolates from the USA carried a phage in this insertion site. In contrast, the majority of isolates from the USA had an occupied wrbA (61%) while this site was rarely occupied in isolates from Australia (2%). 

Significance: This study shows that E. coli O157 from different countries vary in stx subtypes and chromosomal insertion sites of stx-bacteriophages. Such differences may be one factor involved in the different human epidemiology observed in Australia and the USA.