Association between Lactococcus cremoris subsp. cremoris FC intake and microbiota profiles in relation to bowel movement: a post hoc analysis of a placebo-controlled, parallel-group trial

Lactococcus cremoris subsp. cremoris FC (L. cremoris FC) improves bowel movement; however, its associations with the intestinal microbiota remain unclear. To address this gap, this post hoc analysis of a four-week, placebo-controlled, parallel-group trial evaluates the associations between L. cremoris FC intake and gut microbiota profiles. We analyzed fecal samples from 83 healthy Japanese adults with mild constipation who received either placebo capsules containing corn starch (n=20) or L. cremoris FC capsules (50, 75, or 100 mg; n=21 each). Using 16S rRNA gene amplicon sequencing, we evaluated diversity and relative abundance and explored association patterns through Sparse InversE Covariance estimation for Ecological Association and Statistical Inference network analysis. Although no dose dependence was observed, alpha diversity decreased significantly in the L. cremoris FC 100 mg group, whereas beta diversity shifted significantly in the 50 mg group. Short-chain fatty acid producing genera, including Blautia, Anaerobutyricum, Anaerostipes, and Agathobaculum, showed higher relative abundances after intervention in both L. cremoris FC intake groups. Moreover, the relative abundance of Blautia was positively correlated with stool characteristics across post-intervention samples. Exploratory network analysis indicated altered statistical associations among bacterial taxa. Blautia and Roseburia showed higher centrality in the L. cremoris FC 100 mg group, whereas Bacteroides and Ruminococcus were more central in the placebo group. These findings suggest that L. cremoris FC intake was associated with coordinated alterations in microbial diversity, genus-level relative abundance, and association patterns, providing ecological insights into microbiota changes accompanying bowel improvement, though underlying mechanisms remain unclear.