Background: Abrupt dietary changes may disrupt gut microbiota populations and lead to gastrointestinal issues. This study aimed to determine the effects of live Bacillus pumilus SG154 or Lacticaseibacillus paracasei 327 postbiotic on fecal characteristics and microbiota populations of dogs following an abrupt diet change. Methods: Twelve healthy adult English pointer dogs (6.38 ± 2.75 yr) were used in a replicated 3 × 3 Latin square design to test the following treatments: (1) placebo (control; 250 mg maltodextrin/d); (2) live B. pumilus [5 × 10⁹ colony-forming units (CFU)/d]; and (3) L. paracasei postbiotic (100 mg; derived from 2 × 10⁹ CFU/d). Each period lasted 42 days, with the diet change occurring on day 28. Fecal samples were scored and analyzed for pH, dry matter content, and microbiota before and 2, 6, 10, and 14 days after the diet change. Results: The abrupt diet change increased (p < 0.01) fecal pH, increased (p < 0.01) the dysbiosis index, decreased (p < 0.0001) fecal dry matter, and led to a large shift in the fecal microbiota community. Fecal scores were lower (p < 0.05) in the B. pumilus group. B. pumilus reduced (p < 0.05) the relative abundance of fecal Prevotella and Muribaculaceae, while both treatments (B. pumilus; L. paracasei) increased (p < 0.05) the relative abundance of fecal Holdemanella. Conclusions: These results suggest that an abrupt diet change leads to large shifts in fecal microbiota and modified fecal characteristics. The supplementation with a B. pumilus probiotic and a L. paracasei postbiotic slightly altered the relative abundance of a few microbial taxa but was unable to attenuate most responses.

Studies in humans and livestock have demonstrated Bacillus coagulans GBI-30, 6086 to have probiotic potential, suggesting that it may alleviate gastrointestinal (GI) distress commonly associated with diet change in dogs. This study aimed to evaluate the effects of B. coagulans GBI-30, 6086 on fecal scores, pH, dry matter (DM) percentage, and microbiota populations of dogs following an abrupt diet change. English Pointer dogs (n = 12; age = 5.9 ± 2.5 yr; body weight = 26.6 ± 6.1 kg) were used in a replicated 3 × 3 Latin square design and fed commercial diets containing no probiotics or prebiotics. The following treatments were administered orally in gelatin capsules before each daily feeding: (1) placebo control (250 mg maltodextrin/day); (2) B. coagulans [low dose; 5 × 10⁸ colony-forming units (CFU)/day]; and (3) B. coagulans (high dose; 2.5 × 10⁹ CFU/day). An extruded kibble diet was fed for 28 days. Dogs were then abruptly switched to a canned diet and fed for 14 days, with fecal samples collected before and 2, 6, 10, and 14 days after diet change. All data were analyzed using the Mixed Models procedure of SAS 9.4, testing the effects of treatment, time, and treatment*time interactions. Treatment*time interactions were not observed, but the abrupt diet change reduced (p < 0.0001) fecal DM content, increased (p < 0.0001) fecal scores and pH, and reduced (p < 0.0001) fecal bacterial species richness and phylogenetic diversity. Diet change also increased (p < 0.001) fecal Bacteroidota, Fusobacteriota, and Proteobacteria, decreased (p < 0.001) fecal Firmicutes, and altered ~40 fecal bacterial genera relative abundances. Diet-induced changes were minimally impacted by B. coagulans, but fecal scores tended to be lower (i.e., firmer stools; p < 0.10), fecal E. coli and Faecalibacterium abundances were greater (p < 0.05), and fecal bacterial phylogenetic diversity was higher (p < 0.05) in dogs supplemented with the low dose than in controls. Our results demonstrate that abruptly transitioning dogs from a kibble to a canned diet negatively influences fecal characteristics and considerably shifts the composition of the fecal microbiota. Supplementation with B. coagulans did not mitigate the diet-induced shifts to fecal characteristics and most of the microbial taxa, although the low dose impacted some microbial taxa. Further investigation into optimal inclusion levels in pet foods is warranted.