Effects of Fermentable Carbohydrates and Dietary P Supply on Bacterial P Incorporation, Activity and Composition in the Gastrointestinal Tract of Pigs

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Fermentable carbohydrates, such as cellulose and pectin, adversely affect nutrient utilisation in pigs, especially at high dietary inclusion levels. There exists little information about the bacterial phosphorus (P) requirement for fermentation of certain carbohydrates in pigs which may affect the P availability of the host animal. However, the bacterial species composition is known to be susceptible to changes in the carbohydrate composition of the diet. Furthermore, bacterial activity may depend on the bacterial P availability in intestinal digesta. Therefore, this study aimed to determine the bacterial P incorporation, activity and species composition in response to fermentable carbohydrates as well as to differing dietary supply of P. Moreover, the influence on the P recovery in ileal digesta and faeces were estimated.
In the first study, the effects of fermentable carbohydrates on P metabolism, the chemical composition of the faecal mixed bacterial mass (MBM) and the microbial activity in the large intestine (LI) of pigs were determined. Eight barrows (BW 36 kg), fitted with simple T cannulas at the distal ileum, were either fed a low-P corn-soybean meal based control diet or 75% of the control diet supplemented with 25% cellulose, starch or pectin. Both pectin and cellulose caused higher faecal than ileal P recoveries. Ileal VFA levels were more pronounced for the starch rather than the cellulose and pectin treatments, whereas pectin resulted in a higher faecal VFA concentration in comparison to starch and cellulose. The nitrogen (N) content of MBM was higher when cellulose was supplemented. Pectin caused a decrease in the P content of the MBM compared with the control. As a result, the N to P ratio was higher for the pectin (N:P=4.33) than for the control treatment (N:P=2.63), while the calcium (Ca):P ratio remained constant for all treatments, suggesting changes in the accumulation of N, P and Ca in MBM, probably due to changes in the species composition and activity of the microflora.
The response of total bacterial cell counts as well as cell counts of Lactobacillus spp., L. reuteri, L. amylovorus/L. sobrius, L. mucosae, Enterococcus spp., E. faecium, E. faecalis, bifidobacteria, Clostridium coccoides cluster, C. leptum cluster, Bacteroides-Prevotella-Porphyrmonas group and Enterobacteriaceae to the fermentable carbohydrates were determined by quantitative realtime PCR in DNA extracts of ileal digesta. Denaturing gradient gel electrophoresis (DGGE) of DNA fragments, generated by PCR targeting total or Lactobacillus spp. 16S rDNA, was used to estimate the bacterial diversity in the ileum. Starch supplementation strongly stimulated the growth of lactobacilli species in the ileum. Cellulose, in turn, enhanced the numbers of bifidobacteria, but reduced the numbers of L. amylovorus/L.sobrius compared with the control. Finally, pectin tended to increase the cell numbers of L. amylovorus/L. sobrius and the Bacteroides-Prevotella-Porphyrmonas group compared with the control. DGGE analysis revealed increased band numbers for total bacteria in the ileum of animals fed the cellulose and starch supplemented diets, while pectin reduced diversity of Lactobacillus spp. compared with the control.
In the second study, two experiments were conducted to evaluate the effects of P level, phytase supplementation and ileal pectin infusion on the chemical composition of the faecal MBM, bacterial enzyme activity and metabolites in pigs. In each experiment, eight barrows (BW 30 kg) were fitted with simple T-cannulas at the distal ileum. In Exp. 1, the diets were a low-P corn-soybean meal based control diet (3 g P/kg) or the control diet supplemented with monocalcium phosphate (MCP; 7g P/kg). In Exp. 2, the pigs received the control diet or the control diet supplemented with 1000 FTU phytase/kg. Additionally, 60 g apple-pectin dissolved in 1.8 L demineralised water and 1.8 L demineralised water as control were infused daily via the ileal cannula into the LI, respectively. In Exp. 1, supplemental MCP markedly raised the P and Ca content of the MBM, but tended to decrease the N content and reduced the N:P-ratio. Ileal pectin infusion enhanced the faecal polygalacturonase activity as well as tended to increase the faecal VFA concentration. In Exp. 2, supplemental phytase decreased the P content of the MBM as well as total faecal VFA concentration. Bacterial cellulase activity in faeces was inhibited by phytase and pectin, whereas bacterial polygalacturonase activity in faeces increased in response to pectin infusion.
Based on the results of the present work, it can be concluded that the chemical composition of the MBM can be affected by the dietary carbohydrate composition and large intestinal P availability. Moreover, the present results indicate that fermentation intensity in the GIT of pigs does not only depend on the fermentability of the substrate, but also on the intestinal P availability. Fermentation of cellulose and pectin in the LI caused a net P secretion into the LI in both studies, probably due to meet bacterial P requirements for fermentation. This may be important for the P utilisation of the pig, particularly when the dietary P supply is marginal. Furthermore, the ileal bacterial composition is sensible to changes in the carbohydrate composition of the diet. The use of fermentable carbohydrates warrants further research pertaining to the stabilisation of the bacterial ecosystem in pigs.