Daily supplementation with canine-isolated Limosilactobacillus reuteri ATCC PTA 6127 is well tolerated, improves stool consistency, and modulates gut microbiome in dogs

Probiotics are increasingly incorporated into companion animal diets to support gastrointestinal function, yet strain-specific evidence in dogs remains limited. This study evaluated the safety, tolerability, and gastrointestinal effects of daily supplementation with Limosilactobacillus reuteri PureStrong™ (ATCC PTA 6127) in healthy adult dogs. Sixteen purpose-bred Beagle dogs (8/group; balanced by sex) were enrolled in a randomized, blinded, placebo-controlled trial and received either 1 × 10 8 CFU/d of L. reuteri ATCC PTA 6127 or placebo for 28 days. At study initiation, dogs underwent a standardized transition from a dry diet to a canned wet diet, representing a dietary challenge associated with transient gastrointestinal and microbiome changes. Safety was assessed through clinical observations, body weight, food intake, hematology, and serum biochemistry; gastrointestinal outcomes included daily fecal scoring and periodic measurement of fecal pH, moisture, and microbiome composition (16S rRNA gene sequencing). All dogs completed the study without adverse events, and clinical pathology values remained within reference intervals with no treatment-related differences. Dogs receiving L. reuteri exhibited a lower incidence of poorly formed stools compared with placebo (0.58% vs. 6.33%) and were significantly less likely to be assigned to poorer fecal consistency categories ( β = −1.31, p = 0.033), indicating improved stool quality during supplementation. By Day 28, fecal pH was significantly lower in the probiotic group compared with placebo after baseline adjustment (Δ = −0.22; 95% CI: −0.403 to −0.045; p = 0.018). Microbiome analyses indicated that the transition from dry to wet food was the dominant driver of microbial community restructuring. Against this background, probiotic supplementation was associated with an early increase in fecal L. reuteri abundance and more positive longitudinal changes in several lactic acid bacteria and commensal taxa during the first 2 weeks. Most treatment-associated differences were no longer evident by Day 28. In summary, daily supplementation with L. reuteri ATCC PTA 6127 was safe and well tolerated and was associated with improved fecal consistency, lower fecal pH at Day 28, and transient differences in microbiome adaptation during a dietary challenge. These findings provide strain-specific evidence supporting the potential of ATCC PTA 6127 as a dietary probiotic supplement for gastrointestinal stability in dogs.