In vitro evaluation of the effects of Yucca schidigera and inulin on the fermentation potential of the faecal microbiota of dogs fed diets with low or high protein concentrations

This in vitro study aimed to evaluate the effects of Yucca schidigera powder (YSP) and inulin (IN) on protein fermentation metabolites (short-chain fatty acids [SCFA] and branched-chain fatty acids, phenolic and indolic compounds, biogenic amines, ammonia and pH) by using faecal inocula from dogs fed either a low (L) or a high (H) protein diet (crude protein 201 or 377 g/kg as fed). Four treatments for each diet were evaluated in an in vitro batch culture system over 24 h: (1) control with no addition of substrates; (2) 4 g YSP/l; (3) 5 g IN/l; (4) 4 g YSP/l in combination with 5 g IN/l of faecal culture of dogs fed Diet L or H. Several changes in fermentation metabolites were analysed. Samples incubated with the faecal inocula of dogs fed Diet L produced higher concentrations of total SCFA, propionate (p = 0.001), acetate (p <= 0.001), d-lactate (p = 0.041) and indole (p = 0.003), whereas pH (p = 0.004) was decreased. Supplementation of IN increased the content of putrescine, d- and l-lactate, total SCFA, acetate, propionate, n-butyrate (p <= 0.001) and n-valerate (p = 0.003), while i-valerate, indole and pH (p <= 0.001) were reduced. Ammonia was lower (p = 0.013) in samples with faecal inocula from dogs fed Diet H and further reduced by the addition of IN (p <= 0.001). Samples with faecal inocula from dogs fed Diet L had a fewer quotient of ammonia and total SCFA (p = 0.040). Supplementation of YSP (p = 0.021), IN (p <= 0.001) and YSP in combination with IN (p = 0.047) led to a higher reduction of the quotient of ammonia and total SCFA. In conclusion, dietary protein concentration and the supplementation of IN resulted in a stimulation of fermentation while YSP appeared to have only minor effects.