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The gastrointestinal tract ecosystem is a complex system, which we have to understand in its entirety in order to design strategies to improve intestinal health. In this article, we use a segmentation made by Knudsen et al. (2008) to present the main elements that can be found in the digestive environment and their relationship with gut health. Moreover, we will discuss how they are interrelated with each other and the importance of their balance for optimal intestinal health.
Animal
The digestive tract is a key element of gut health, which can be divided into different parts. To begin with, there is the mucus layer. That is, a glycocalyx rich in mucin, excreted by the goblet cells of the intestine. This mucus, in turn, is full of antimicrobial peptides excreted by the epithelial cells and will protect against the bacteria (commensal and pathogenic) that reside in the intestine.
Just below, there is the epithelium of enterocytes. It is formed by a cellular monolayer membrane of vital importance, as it is responsible not only for absorbing all the nutrients (because they cover all the microvilli), but also for releasing enzymes that participate both in the digestion and in the innate immunity of the organism. In addition, at a physical level, this cellular membrane is the first barrier of the organism against pathogens present in the intestinal lumen (what we know as the intestinal barrier). When designing strategies to protect this epithelium, it is important to keep animal physiology in mind. During the first hours of life, piglets epithelial cell barrier is (and must be) lax, allowing the translocation of antibodies and immune cells from the colostrum between the cells. This fact permits newborn piglets to have a certain degree of passive protection against pathogens, even though they have an underdeveloped immune system. Later on, the cellular junctions (known as tight junctions) must become stronger and form an authentic "selective" barrier, which prevents the entry of pathogens, but allows the entry of nutrients.
Finally, there is the gut-associated lymphoid tissue (GALT), which forms the intestine's immune system together with the epithelial barrier. It must be mentioned that the intestine, as an immune organ, has particularities that make it unique. For example, it is the largest organ of the body with immune functions. Nevertheless, it can physiologically tolerate a large number of antigens from the diet and microbiota (Burkey et al., 2009).
Microbiota
Phylogenetic studies have determined that the gastrointestinal tract of mammals contain one order more of bacteria than cells present in the body of the host; among which there are beneficial, commensal and pathogenic bacteria. This bacterial component has been described as an "indispensable organ" in humans (Forysthe et al., 2015), which contributes to a plethora of genetic information present, but not native to its host.
In porcine, we know that these microorganisms have a very important role in the metabolism of nutrients. In fact, it has recently been reported that having an enterotype or another (in other words, a profile or another of microbiota) is directly related to the feed conversion rate of the animals (Ramayo-Caldas et al., 2016). Moreover, microbiota is also extremely important for gut health. For example, it is beneficial to have butyrogenic bacteria (producers of butyrate), since butyrate is the principal source of energy for the enterocytes. Alternatively, bacteria that ferment to other short chain fatty acids such as acetate or propionate may also be interesting for their energetic and antimicrobial functions.
Diet
When we feed our pigs, we not only provide nutrients to our animals, but also (and largely) provide the necessary substrate for their microbiota. Consequently, diet plays a very significant role in intestinal health, because by including certain ingredients (fermentable fibers) or functional additives (prebiotics, organic acids...) we can directly act into intestinal health promoting a specific microbiota and fermentation profile.
In conclusion, the presented elements form a triad of interrelated components that must be balanced in our animals to achieve a good gut health context. A problem in one of these elements, such as the rupture of the epithelial membrane of enterocytes, or a sudden alteration in the fermentation profile of the diet; will create a situation of instability, in turn affecting animal’s efficiency and productivity. Besides, these problems very often lead to a situation of gastrointestinal pathology, where we will have to invest money in a therapy and may have some deaths among our animals. In the following articles, we will present the strategies that can be used to strengthen this balance, that is, to promote gut health.
