Technical Abstract: Citrus by-products contain essential oils that possess antimicrobial activities that can exert damage to the cell wall of gram-negative bacteria. This alteration to gram-negative microbes has resulted in CBP being investigated as a potential pre-harvest pathogen intervention strategy to reduce Shiga toxin-producing Escherichia coli (STEC) populations. Although CBP research has been conducted utilizing O157:H7, there is limited knowledge as to the effect, if any, that CBP may have on the viability of other Shiga toxin-producing E. coli (STEC) serogroups. Traditionally, researchers have believed that non-O157 serogroups grow similarly to O157:H7. However, a study recently conducted within our group suggests that the responses of these non-O157 serogroups are not universal. Therefore, our hypothesis was that CBP might decrease STEC populations with varying degrees between non-O157 STEC serogroups when cultured within ruminal microbial fermentations (RMF). Variations were apparent in the populations of the various STEC (log10 CFU/mL) within RMF supplemented with CBP. CBP decreased populations of O103:H8 and O157:H7 dealta-stx1 stx2, O103:H8 approximately 1 log10 CFU/ml reduction over the 6 h study in the presence of >5% powdered CBP. When 10% and 20% powdered CBP was added to the RMF, populations of O157:H7 dealta-stx1 stx2 decreased by 1.5 log10 and 5 log10 CFU/ml, respectively. Although our research has reported a decrease in pH values with increasing CBP concentrations and an observed decrease in O103:H8 and O157:H7 delta-stx1stx2 populations, this trend was not observed in other STEC. Further research is needed to determine how the various STEC survive within RMF when supplemented with CBP.