Effects of Inulin Chain Length on Fermentation by Equine Fecal Bacteria and Streptococcus bovis

Grass fructans can be fermented by Gram-positive bacteria (e.g., Streptococcus bovis) in the equine hindgut, increasing production of lactic acid and decreasing pH. The degree of polymerization (DP) of fructans has been suggested to influence fermentation rates. The objective of the present study was to determine how DP impacts fermentation by equine fecal bacteria and a model S. bovis. Fecal microbes from three mares were harvested by differential centrifugation, washed, and resuspended in anaerobic media containing short-chain (SC; DP ≤ 10) or long-chain (LC) inulin (DP ≥ 23) from 0% to 2% wt/vol. After 24 hours of incubation (37°C), samples were collected for pH determination. Data were analyzed using the general linear models (GLM) procedure testing for the effect of treatment, concentration, and treatment × concentration (SAS v. 9.3). At all concentrations, the pH was lower in SC fermentations than in LC (P < .0001, in all cases). To determine the effect of DP on S. bovis, cultures were grown (39°C, 9 hours) with 0.1%, 0.5%, or 1.3% SC or LC inulin. Optical density (600 nm) was determined by spectrophotometry. Maximum specific growth rates (μ) were determined by linear regression (2-5 hours). Data were analyzed using the one-way analysis of variance procedure (SAS v. 9.3). The final optical density (600 nm), μ, and yield were higher with SC than with LC fermentation (P < .05). These results indicate that SC inulin may be more available for fermentation than LC inulin by equine fecal bacteria and S. bovis, specifically.