182. Effects of Low-dose Antimicrobial Feeding on the Ruminant Gastrointestinal Tract Bile Acid Profile over Time.

Abstract A more mechanistic understanding of how antimicrobial supplementation (Tylosin plus Monensin) affects the ruminant gastrointestinal tract (GIT) in high grain diets may help accelerate the development of effective antibiotic alternatives. Previous work in swine has demonstrated that tylosin supplementation alters bile acid (BA) profiles, suggesting that microbiome-driven BA metabolism may be an important pathway through which antimicrobials influence host-microbial interactions in the GIT. Therefore, the objective of this study was to determine how low-dose antimicrobial supplementation influences BA profiles in the ruminant GIT over time. A serial slaughter study was conducted using 72 steers (304.2 ± 36.7 kg initial BW). All steers were fed a high-forage diet pre-study to mimic industry backgrounding conditions, and eight were harvested on day 0 as a baseline. The remaining steers were individually housed, fed a 93% concentrate diet, and beginning on day 1, steers were assigned to one of two dietary treatments: 1) control, without antibiotics, or 2) 75 mg tylosin and 200 mg monensin. Sixteen steers were harvested on day 8, 22, 92, and 172 of the finishing period (n = 8 per treatment per harvest date). At slaughter, samples were obtained from the bile, jejunum, and cecum. Bile acids were extracted from samples with a solid-phase extraction protocol and then analyzed by liquid chromatography-mass spectrometry. Data were analyzed using SAS PROC GLIMMIX on a percentage basis. In bile, the proportion of total tauro-conjugated BA and primary tauro-conjugated BA was greater in steers supplemented with Tylosin plus Monensin (T+M) compared with control steers on day 8 (P < 0.05). The proportion of secondary tauro-conjugated BA was also greater in T+M steers on days 8 and 22 (P < 0.05). In contrast, the percentage of primary deconjugated BA was lower in T+M compared to control steers (P < 0.05). In jejunum, on day 8, steers supplemented with T+M had a greater proportion of total conjugated BA, driven primarily by increases in tauro-conjugated BA, including primary tauro-conjugated BA (P < 0.05), which corresponded with the decrease in total and secondary deconjugated BA in T+M steers on day 8 (P < 0.05). No differences were observed in cecal BA composition by class between treatments (P > 0.05). Taken together, these results suggest that T+M alter the BA profile in bile and the jejunum during early adaptation to a high-grain diet, and that these effects are maintained through the first 92 days of the feeding period, likely through reduced microbial deconjugation activity. This shift results in greater proportions of conjugated BA and lower proportions of deconjugated BA, which is associated with a less cytotoxic and hydrophobic BA pool and may support improved intestinal health.