In vitro fermentation characteristics of dietary fibers using fecal inoculum from dogs consuming commercial or grain kefir

Traditional grain kefir is produced from the fermentation of milk with yeast- and bacteria-containing cultures. To maintain consistency and adhere to food safety guidelines, commercial kefir products are based on starter bacterial cultures. Bacterial profiles of starter vs. grain kefirs differ, and their influence on health effects is unknown. Our objectives were to determine the in vitro fermentation characteristics of common dietary fibers using fecal inoculum from dogs supplemented with kefir or kefir bacterial culture as inoculum. Healthy adult dogs were allotted to one of 3 treatments and supplemented for 14 d (n = 4/treatment): 1) 2% reduced-fat milk treated with lactase (CNTL), 2) starter kefir (S-Kefir), or 3) grain kefir (G-Kefir). After 14 d, fresh fecal samples were collected and frozen in a 20% glycerol solution. For the in vitro experiment, fecal samples were thawed, diluted in an anaerobic diluting solution, and used to inoculate tubes containing semi-defined medium and either cellulose (CEL), pectin (PC), beet pulp (BP), or chicory pulp (CP). Tubes were incubated for 0, 6, 12, or 18 h, with short-chain fatty acids (SCFA), pH, and microbiota measured at each time point. A second in vitro experiment was conducted using similar methods and measurements but with S-Kefir and G-Kefir as inoculum sources. Effects of treatment (inoculum), time, and treatment*time interactions within the fiber source were analyzed statistically using Mixed Models and repeated measures, with P < 0.05 being significant. Using fecal inoculum, BP and PC were rapidly fermented, leading to large pH reductions, SCFA increases, and microbiota shifts. pH change was of greater (P < 0.05) magnitude (PC) and higher (P < 0.05) kinetic rate (CP) when using feces from dogs fed S-Kefir or G-Kefir than controls. Butyrate increases were greater (P < 0.05) in tubes inoculated with G-Kefir feces than in S-Kefir or control feces. When PC and BP were fermented, tubes with S-Kefir feces had greater (P < 0.05) acetate, propionate, and total SCFA increases than G-Kefir or control feces. Fermentations were slower when using kefir cultures as inoculum, but some differences were noted. Bacterial beta diversity and relative abundances shifted over time within each substrate and were unique to the inoculum source. Our data suggest that the activity of kefir bacterial populations differs and that kefir consumption changes the abundance and activity of the fecal microbiota of dogs, justifying in vivo investigation.