Impact of Early Weaning on Development of the Swine Gut Microbiome

Abstract

Considering that pigs are naturally weaned between 12 and 18 weeks of age, the common practice in the modern swine industry of weaning as early as between two and four weeks of age increases challenges during this transition period. Indeed, young pigs with an immature gut are suddenly separated from the sow, switched from milk to a diet consisting of only solid ingredients, and subjected to a new social hierarchy from mixing multiple litters. From the perspective of host gut development, weaning under these conditions causes a regression in histological structure as well as in digestive and barrier functions. While the gut is the main center of immunity in mature animals, the underdeveloped gut of early weaned pigs has yet to contribute to this function until seven weeks of age. The gut microbiota or microbiome, an essential contributor to the health and nutrition of their animal host, undergoes dramatic alterations during this transition, and this descriptive review aims to present a microbial ecology-based perspective on these events. Indeed, as gut microbial communities are dependent on cross-feeding relationships, the change in substrate availability triggers a cascade of succession events until a stable composition is reached. During this process, the gut microbiota is unstable and prone to dysbiosis, which can devolve into a diseased state. One potential strategy to accelerate maturation of the gut microbiome would be to identify microbial species that are critical to mature swine gut microbiomes, and develop strategies to facilitate their establishment in early post-weaning microbial communities.

Keywords: gut development; microbiome; swine; weaning.

Figure 1

Hypothetical model of micro-habitat formation and microbial succession. (A) The surface protein ‘glycoprotein 1’, which is expressed by epithelial cells, can be metabolized by a specialized microbial species (‘glycoprotein utilizer 1’) to generate a particular compound (‘end product 1’). (B) As ‘glycoprotein utilizer 1’ proliferates, there is an accumulation of ‘end product 1’. (C) The compound ‘end product 1’ is in turn metabolized into short-chain fatty acids (SCFAs) by a specialized cross-feeding species (‘end product 1 utilizer’). (D) As ‘end product 1 utilizer’ proliferates, there is an accumulation of SCFAs, which are absorbed by epithelial cells. (E) As a result of SCFA absorption, epithelial cells switch from expressing ‘glycoprotein 1’ to expressing ‘glycoprotein 2’; this change in glycoprotein expression results in a complete transformation of the bacterial community composition, from primary utilizers to cross-feeding species. (F) An additional layer of complexity comes from substrates provided by the diet (‘polymer substrate’), which result in the proliferation of other microbial specialists that each can metabolize a particular type of dietary substrate. These utilizers of dietary substrates can also support other consortia of cross-feeding species (not represented in this diagram). (G) Figure legend.

Figure 2

Factors that impact gut microbial community assembly and composition. Diet is the most influential factor affecting the assembly and composition of gut microbial communities in young pigs (A); milk (nursing phase—blue arrow) and plant-based ingredients, such as corn and soybean meal (post-weaning phase—orange arrow), provide very different types of substrates that each support distinct types of metabolic activities. The young pig’s environment contains different microbiomes (B), such as those found on the sow’s teats, milk, and feces, which act as a source of microorganisms that can colonize the gut. The developing gastrointestinal tract also plays an important role, such as (C) through the expression of host factors, which include mucins (muc) and cell-bound glycoproteins (gp).