In pigs, the microbial ecosystem of the gastrointestinal tract (GIT) is influenced by various factors; however, variations in diet composition have been identified as one of the most important determinants. Marked changes in fermentation activities and microbial ecology may occur when altering the diet, for example, from milk to solid feed during weaning. In that way, access of pathogens to the disturbed ecosystem is alleviated, leading to infectious diseases and diarrhea. Thus, there is increasing interest in improving intestinal health by use of dietary ingredients suitable to beneficially affect the microbial composition and activity. For example, fermentable carbohydrates have been shown to promote growth of beneficial Lactobacillus species and bifidobacteria, thereby enhancing colonization resistance against potential pathogens or production of short-chain fatty acids, which can be used as energy source for epithelial cells. On the other hand, fermentation of protein results in the production of various potentially toxic products, such as amines and NH3, and is often associated with growth of potential pathogens. In that way, excessive protein intake has been shown to stimulate the growth of potentially pathogenic species such as Clostridium perfringens, and to reduce fecal counts of beneficial bifidobacteria. Therefore, it seems to be a promising approach to support growth and metabolic activity of the beneficial microbiota by developing suitable feeding strategies. For example, a reduction of dietary CP content and, at the same time, dietary supplementation with fermentable carbohydrates have proven to successfully suppress protein fermentation. In addition, the intestinal microbiota seems to be sensible to variations in dietary protein source, such as the use of highly digestible protein sources may reduce growth of protein-fermenting and potentially pathogenic species. The objective of the present review is to assess the impact of dietary protein on microbiota composition and activity in the GIT of piglets. Attention will be given to studies designed to determine the effect of variations in total protein supply, protein source and supplementation of fermentable carbohydrates to the diet on composition and metabolic activity of the intestinal microbiota.

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A study with growing barrows was conducted to evaluate of variations in particle size and degree of heat treatment during processing on standardized ileal digestibility (SID) of AA in soybean (Glycine max) meal (SBM). A commercial SBM batch was visually identified as being overtoasted due to its brownish color and was separated into small and large particles using a 1-mm sieve. In addition, 3 SBM were produced from 1 batch of soybean and exposed to different processing conditions (temperature and direct steam contact) referred to as mild (105°C; 34 min), medium (115°C; 45 min), and strong (139°C; 7 min). In total, 5 SBM-corn (Zea mays) starch-based diets were formulated to contain SBM as the sole protein source. This experiment was conducted according to a 6 × 6 Latin square design using 6 barrows (23 kg initial BW) fitted with a T-cannula at the distal ileum. With increasing particle size, SID of His and some dispensable AA increased (P < 0.05). Lower SID values in small compared to large SBM particles indicate more pronounced heat damage possibly due to increased surface area. The SID of CP and AA was lowest in the mild, intermediate in the strong, and highest in the medium toasted SBM (P < 0.001). These differences in SID are reflected in varying contents of trypsin inhibitors, Lys, reactive Lys, and NDF. In conclusion, both differences in particles size and variations in thermal processing conditions of SBM may affect SID of CP and AA.

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In piglets, the development and composition of intestinal microbiota is influenced by dietary factors. A considerable reduction in feed intake during the transition from liquid to solid feed is often accompanied by postweaning diarrhea. Therefore, it is hypothesized that variations in feeding level during weaning may affect intestinal microbial composition. Forty-eight piglets fitted with simple ileal T-cannulas were used to examine the effects of a high (60 g/kg BW) and a low (30 g/kg BW) feeding level on the composition of the ileal and fecal microbiota. The assay diets contained graded inclusion levels of soybean (Glycine max) meal or casein. Bacterial cell numbers of total eubacteria, Lactobacillus spp., and the mainly proteolytic Clostridium leptum, Clostridium coccoides, Enterobacteriaceae, and Bacteroides-Prevotella-Porphyromonas group (Bacteroides group) of feces and ileal digesta were determined by use of real-time PCR. There were no interactions between feeding level and protein source except for the Bacteroides group in ileal digesta. Ileal cell numbers of lactobacilli were increased (P < 0.001) at the higher feeding level. In contrast, ileal cell numbers of Clostridium coccoides were lower (P < 0.001) at the higher feeding level. There were no differences of bacterial cell numbers in feces. Results indicate that feeding level affects microbial composition in the small intestine. Also, sufficient feed intake during weaning encourages proliferation of beneficial bacteria, thereby contributing to improved gut health.