Traditionally the digestive tract of pigs has been considered in view of its primary function being the site where ingested nutrients are processed and enzymatically degraded or fermented and nutrients are absorbed from the lumen towards systemic organs and tissues. More recently the gut is also being considered more explicitly for its other functions, including its barrier function. The barrier function comprises different elements in which the intestinal microbiota and the local immune system are important components. Gut health can be defined as the functional status of the gut in relation to its capacity to exert its various functions to allow the animal to achieve its potential productive performance under a variety of conditions. The gut has a residing microbiota which increases in complexity and density going from the proximal to distal part. In man, mammals and avian species the colonisation starts at births or hatching and develops into a more mature microbiome during later life consisting of more than 1000 species (Rajilic-Stojanovic et al., 2007; Zoetendal et al., 2008; Isaacson and Kim, 2012). There are distinct differences in the composition of the pig intestinal microbiome moving from the proximal end of the intestinal tract to the distal end. The majority (>90%) of the bacteria in the pig intestinal microbiome are from two phyla: Firmicutes and Bacteroidetes. However, the ileum has a high percentage of bacteria in the phylum Proteobacterium (up to 40%) (Isaacson and Kim, 2012). The process of colonisation and microbial succession in the post-natal period is affected by microbiota in the environment, genotype of pigs, animal management (housing and use of antibiotics) and by diet composition in terms of nutrient and ingredient composition and presence of specific feed additives. The microbiota play an important role in the establishment and maintenance of an efficient barrier. A significant part of the immune system (up to 70-75% of the immune cells) is also localized in the mucosa and submucosa in the gut. In the gut tissue programming and regulation of the local immune system occurs. The local immune system in the gut also communicates with other parts of the system elsewhere in the body (Weng en Walker, 2013) and is involved in the development of immune competence. Cross talk between the gut microbiota and the mucosal immune system in the gut plays an important role in the development of the local immune system and the development of immune competence.
Over the last years both in human and animal research the importance of early microbial colonisation and development of the gut on the long term health and performance has been established. In man, for example, microbiota composition in the gut has been associated with development of immune related diseases, obesity and development of inflammatory bowel disease (IBD). In piglets, up to more recently, attention was primarily given to manipulation of microbiota composition in the post weaning period.
More recently, also the postnatal period has been identified as a critical window allowing manipulation of intestinal microbiota composition with longer lasting effects. In the framework of the FP7 project INTERPLAY a model was developed in which the effects of early microbial association in postnatal piglets and the effects of modifying diet composition (either of not provision of a diet containing (soya/palm oil or coconut oil as MCT) were studied (Jansman et al. 2012; 2015). These studies revealed that early association and manipulation of diet composition can have lasting effects on intestinal microbiota composition with functional effects on the development the local immune system in the gut.
It can be concluded that both the postnatal and post weaning periods are critical windows for influencing the intestinal microbiota composition with functional consequences for the development of the various functions of the gut, including its barrier and immunological functions. More attention should be given to the nutritional options and mechanisms involved and the long term consequences of manipulating the intestinal microbiota on health and performance.
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