FEEDING THE MICROBIOME

Richard Ducatelle, Evy Goossens, Venessa Eeckhaut & Filip Van Immerseel
Livestock Gut Health Team, Faculty of Veterinary Medicine; Ghent University, Belgium
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INTRODUCTION
Bacteria have been on this planet for millions of years before the vertebrates evolved. Thus vertebrates emerged in a world that was already conquered by the microbes. When considering the animals microbiome, we are thus looking at millions of years of coevolution. It is therefore not surprising that the interactions between both kingdoms are extremely complex.

When considering, however, that the building blocks of a bacterial cell (i.e. amino acids, nucleic acids, lipids, trace minerals, etc) are remarkably similar to the building blocks of an animal or a human cell, it can be inferred that both bacteria and higher organisms compete for the same nutrients. In order to avoid such competition, absorption of nutrients in higher organisms occurs in a secluded space, namely the intestinal tract. Indeed, in the small intestine, where the actual digestion and absorption takes place, a large spectrum of control mechanisms keeps the density of bacteria remarkably low, mostly below 103 per gram. In the caeca of birds and in the caecum and colon of mammals, however, bacteria are not only tolerated but even they are supported and fed by the host, through the secretion of large amounts of mucins. These mucins are used as nutrient source by specialized bacterial species such as Akkermansia muciniphila, which cross-feed to other bacterial species in order to preserve the individuals core microbiome during episodes of starvation (Such et al., 2019). When feed is available, most of the digestible fraction in absorbed in the small intestine, thus leaving only the undigestible fraction for the microbes that live in the lower intestinal tract. Expansion of the microbial population in the small intestine, especially in the duodenum and jejunum, is inhibited by powerful host antibacterial defense mechanisms which include not only specific anti-bacterial peptides and proteins, but also non-specific inhibiting substances such as the acid produced in the proventriculus. The latter can be bactericidal but it can also induce the ‘viable-but-non-culturable-state’ (Geirnaert et al., 2014), allowing bacteria to pass through the small intestine in a metabolically inactive state.

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