| Summary: | Reducing <i>Salmonella</i> in cattle may mitigate the risk of transmission through the food chain. <i>Megasphaera elsdenii</i> (ME) is a microorganism found naturally in the bovine rumen that can be administered as a probiotic to mitigate ruminal acidosis. Understanding the impact of feeding ME to <i>Salmonella</i> populations in cattle was the objective of this study. Bovine ruminal fluid (RF) and feces were inoculated with antibiotic susceptible or resistant <i>Salmonella</i> and treated with varying concentrations of ME. <i>Salmonella</i> was enumerated at 0, 24, 48, and 72 h using the most probable number (MPN). Volatile fatty acids (VFAs) and pH were recorded from non-inoculated samples. Treating RF with ME did not significantly impact <i>Salmonella</i> concentration or VFA production (<i>p</i> > 0.05). The pH of RF and feces decreased over time (<i>p</i> ≤ 0.05). <i>Salmonella</i> concentration declined in feces, with the largest reduction of 1.92 log MPN/g and 1.05 log MPN/g observed for antibiotic susceptible <i>Salmonella</i> between 0 and 72 h by the 2.5 × 10<sup>5</sup> CFU/g and control (0.0 CFU/g) concentration of ME, respectively. Treating RF with ME did not impact <i>Salmonella</i> concentration. <i>Salmonella</i> concentration in feces decreased, although ME must be further investigated before a conclusion regarding efficacy in vitro can be determined.
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