13 14 Running title: Metabolic activity of piglet gut microbiota 15 2 ORIGINALITY -SIGNIFICANCE STATEMENT 16Bacterial metabolites are key molecular intermediates between the gut microbiota and host cells. Our 17 study in piglets reveals that the metabolic activity of the gut microbiota shifts at weaning, a key 18 developmental period for intestinal and immune health. We also show that this functional maturation 19 of the gut microbiota is strongly influenced by maternal environment. Thus, targeting early life 20 environmental factors is a promising strategy to program health trough the production of beneficial 21 bacterial metabolites at the suckling-to-weaning transition. 22 from the second maternity farm probably because they already had high relative concentration of short 34 chain fatty acids and higher relative abundance of Christensenellaceae R-7 group and 35Ruminococcaceae UCG-002 during the suckling period. In conclusion, the functional maturation of 36 the microbiota at weaning is highly dependent on the maternal environment in piglets. 37
38
KEY WORDS 39Early life, Milk, Solid food, Metabolites 40 observed only in piglets from one of the two maternity farms studied. In piglets from maternity 1, 126 there was a reduction of the relative abundance of Proteobacteria, Enterobacteriaceae and Escherichia-127Shigella after weaning while the relative abundances of Christensenellaceae, Christensenellaceae R-7 128 group and Ruminococcaceae UCG-002 increased. In piglets from maternity 2, the relative abundance 129 of Peptostreptococcaceae increased after weaning. After weaning (at day 23), the relative abundances 130 of Spirochaetes and Ruminococcaceae UCG-002 were higher in piglets from maternity 2 when 131 compared to piglets from maternity 1 while the opposite was observed for Atopobiaceae. Altogether, 132these results show that both maternal environment and weaning shape the microbiota composition in 133 piglets. 134 135 ACKNOWLEDGMENTS 369