Summary: | The present study evaluated gluco-conjugation as a measure to delay thymol absorption and enhance its antimicrobial activity in the gut of weaned piglets. The three dietary treatments consisted of a basal diet without additives (T<sub>CON</sub>), supplemented with thymol at 3.7 mmol/kg dry matter (T<sub>THY</sub>), or with an equimolar amount of thymol α-D-glucopyranoside (T<sub>TαG</sub>). Each dietary treatment was replicated in 6 pens with 2 piglets per pen (<i>n</i> = 12 for analytical parameters) and was supplemented for 14 days. The total (free plus gluco-conjugated) thymol concentrations in the stomach contents were 14% lower in T<sub>TαG</sub> as compared to T<sub>THY</sub> piglets. Neither of the additives could be detected further down the gut. <i>E.coli</i> counts in the proximal small intestine were significantly lower in T<sub>THY</sub> than in T<sub>TαG</sub> pigs (3.35 vs. 4.29 log<sub>10</sub> CFU/g); however, other bacterial counts and their metabolites were unaffected by treatment. A metagenomic bacterial analysis revealed a great relative abundance of <i>Lactobacillus</i> spp. in the distal small intestine (range 88.4−99.9%), irrespective of treatment. The intestinal barrier function was improved by T<sub>THY</sub>, but not T<sub>TαG</sub>, compared to T<sub>CON.</sub> In conclusion, gluco-conjugation did not result in higher thymol concentrations in the gut, but conversely, it seemed to diminish the biological effects of thymol in vivo.
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