Citrus byproducts (CBPs) are utilized as a low cost nutritional supplement to the diets of cattle and have been suggested to inhibit the growth of both Escherichia coli O157:H7 and Salmonella. The objective of this study was to examine the effects in vitro that varying concentrations of CBP in the powdered or pelleted variety have on the survival of Shiga-toxin Escherichia coli (STEC) serotypes O26:H11, O103:H8, O111:H8, O145:H28, and O157:H7 in bovine ruminal microorganism media. The O26:H11, O111:H8, O145:H28, and O157:H7 serotypes did not exhibit a change in populations in media supplemented with CBP with either variety. The O103:H8 serotype displayed a general trend for an approximate reduction in 5% powdered CBP and 20% pelleted CBP over 6？h. There was a trend for reductions in populations of a variant form of O157:H7 mutated in the stx1 and stx2 genes in higher concentrations of CBP. These results suggest that variations exist in the survival of these serotypes of STEC within mixed ruminal microorganism fluid media when supplemented with CBP. Further research is needed to determine why CBPs affect STEC serotypes differently. 1. Introduction Shiga-toxin producing Escherichia coli (STEC) is capable of naturally colonizing within the gastrointestinal tract of cattle without causing illness [1]. Human consumption of products contaminated with STEC can cause the severe illnesses hemorrhagic colitis and hemorrhagic uremic syndrome [2, 3]. The most notorious STEC within the meat industry has been E. coli O157:H7. Due to increased surveillance and pre- and post-harvest intervention, the occurrence of O157:H7 infections in the United States has been reduced to ≤1 case per 100,000 people. However, there now appears to be an increase in the occurrence of foodborne outbreaks due to non-O157 STEC serotypes. According to the Center for Disease Control (CDC) an estimated 265,000 cases of STEC infections were reported a year; of these, approximately 67% are attributed to non-O157 STEC [4]. With increased concerns related to the prevalence of non-O157 outbreaks, the United States Department of Agriculture Food Safety and Inspection Service (USDA-FSIS) has recently labeled the non-O157 STEC serogroups O26, O45, O103, O111, O121, and O145 as adulterants in fresh nonintact beef products [5]. The production of citrus for various food and nonfood products generates byproducts, such as the pulp and peel from citrus fruit. These citrus byproducts (CBPs) have been utilized by dairy and beef cattle producers in some regions of the United States as an inexpensive
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