Isolation of Escherichia coli O157:H7 strains that do not produce Shiga toxin from bovine, avian and environmental sources.

Objective: To determine the potential for naturally occurring Shiga toxin-negative Escherichia coli O157 to acquire stx(2) genes.

Results: Multiple E. coli O157:H7 isolates positive for eae and ehxA, but not for stx genes, were isolated from cattle, water trough sediment, animal bedding and wild bird sources on several Ohio dairy farms. These isolates were experimentally lysogenized by stx(2)-converting bacteriophage.

Conclusions: Shiga toxin-negative strains of E. coli O157 are present in multiple animal and environmental sources. Conclusions: Shiga toxin-negative strains of E. coli O157 present in the food production environment are able to acquire the stx genes, demonstrating their potential to emerge as new Shiga toxin-producing E. coli strains.