177 Effects of low-dose antimicrobial feeding on the ruminant gastrointestinal tract bile acid and lipidome profile.

Abstract Monensin, tylosin, and chlortetracycline (CTC) are commonly used antimicrobials in high-grain feedlot cattle diets to promote growth. However, growing concerns regarding antibiotic resistance highlight the need for alternative strategies. A more mechanistic understanding of how antimicrobial drugs affect metabolites in the ruminant gastrointestinal tract (GIT) may help develop effective alternatives. Therefore, this study aims to investigate how antimicrobial drugs influence the bile acid (BA) and lipidome profile in the ruminant gastrointestinal tract. Thirty steers (355 ± 16.8 kg) were individually housed during the finishing phase, fed a 93% concentrate diet for 148 days and supplemented with one of three dietary antibiotic treatments starting on day 0 (10/treatment): 1) control, without antibiotics, 2) 75 mg of tylosin and 200 mg of monensin, and 3) 70 mg of CTC per head per day. At slaughter, samples were obtained from the rumen, jejunum, cecum, and feces. Bile acids were extracted from samples with a solid-phase extraction (SPE) protocol and then analyzed by liquid chromatography-mass spectrometry. Lipids were extracted by the Bligh & Dyer protocol and then screened by mass spectrometry. Data were analyzed using SAS PROC GLIMMIX and Principal Component Analysis in MetaboAnalyst 6.0. The bile acid profile differed across all locations (P < 0.05). In the jejunum, the total concentration of conjugated primary BA was greater (P = 0.04) in steers fed Tylosin plus Monensin (T+M) compared to CTC or control, while the total concentration of secondary deconjugated BA did not differ among treatments (P = 0.91). Taurolithocholic and Glycoursodeoxycholic acid were greater (P < 0.05) in the T+M group compared to CTC and control in the jejunum. In the cecum, the concentrations of Taurocholic and Taurochenodeoxycholic acid were increased (P < 0.05) in steers fed T+M compared to CTC and control. Fecal BA concentration did not differ for total or individual bile acids (P ≥ 0.210). Lipidomic profiles differed (P < 0.05) across all sampled locations. In jejunum and feces, both antibiotic treatments had a different lipid profile compared to the control (P < 0.05). In the rumen, lipid profiles from T+M steers differed from CTC and control (P < 0.05). Lipid profiles from the cecum of CTC and T+M treated steers tended to differ from the control steers (P < 0.10). In conclusion, T+M decreased the microbial modification of primary conjugated BA to secondary deconjugated BA in the jejunum, with BA modifications noted in the cecum but no differences in feces. Lipidomic profiles were also affected by antibiotic treatments across the gastrointestinal tract highlighting their impact on nutrient metabolism.