Limosilactobacillus fermentum LF61: A multidimensional study on safety and functionality from genomics to clinical application.

This study presents a comprehensive multidimensional assessment of the safety and functional efficacy of Limosilactobacillus fermentum LF61, a strain isolated from human milk. Genomic analysis revealed no virulence factors (VFDB), drug resistance genes (CARD), or toxin synthesis gene cluster (antiSMASH) within its chromosome (2.04 Mb) and plasmid (15.5 kb), meeting EFSA's QPS safety criteria. In vitro studies demonstrated that LF61 exhibited a 2-h survival rate of >98% in gastric acid (pH 2.0) and a survival rate of 99.66% in intestinal fluid (pH 8.0). LF61 was also nontoxic to Caco-2 cells (metabolic activity at 20% concentration: 100.3 ± 2.1%). An acute oral toxicity test (in ICR mice) demonstrated an LD50 > 2 × 10 CFU/kg. In a randomized, double-blind clinical trial (n = 49), daily intake of 3 × 10 CFU of LF61 for 8 weeks increased serum levels of the antimicrobial peptide LL-37 by 12.3% (p < 0.05), and IgA, IgG, and IgM by 18.7%, 15.2%, and 9.8%, respectively (p < 0.05). Metagenomic analysis revealed that LF61 promoted colonization by short-chain fatty acid-producing bacteria, such as Mitsuokella and Turicibacter (LDA >3), activated the carbohydrate metabolism pathway (p = 0.002), and maintained stable α-diversity in the microbiome (Shannon index p > 0.05). Collectively, our findings indicate that LF61 exerts beneficial effects via a gut-immune axis bidirectional regulatory mechanism, offering a theoretical basis and clinical evidence for the development of novel immunomodulatory probiotics targeting the gut-immune axis.