ABSTRACT The objective of this research was to determine the impact of phytochemical addition on gastrointestinal fermentation end-products and foodborne pathogen growth in in vitro fermentations. In order to test these objectives, four experiments were conducted using both anaerobic and aerobic growth techniques. The first two studies were performed using specific rumen fluid inoculum, from goats selected for their high or low consumption preferences of plants (juniper) containing high levels of phytochemicals. The first study, investigated the impact of a specific level of a common monoterpene (camphor) found in plants throughout the world over time, whereas a second study that observed the effect of camphor concentrations on fermentation end-products. Ruminal fluid from goats exhibited a dietary impact on short-chain fatty acid concentration with juniper free diets exhibiting greater SCFA production. Additionally, the microbiome from each group of goats were different, and Ruminococcaceae populations were higher in rumen fluid when goats received a juniper free diet compared to those fed juniper. The third study highlighted microbial end-product dynamics when rumen fluid fermentations were treated with camphor, and different fermentative substrates were added for either 12 or 24 hours of incubation. Camphor had limited effects on rumen microbial fermentation and increased butyrate proportion across all substrate types. The fourth study compared different concentrations of two monoterpenes (camphor and eucalyptol) and the growth of the common foodborne pathogenic bacteria Salmonella enterica serotype Newport in pure culture. The highest concentrations of camphor and eucalyptol (9.85 mM) both halted growth. In conclusion, phytochemicals are impactful on the microbial population of the gut of ruminant animals, and have variable dynamics, emphasizing that further need for in vivo research to complement the in vitro models used in this dissertation.