Safety Assessment of Two Human Fecal Bacteroides Strain Isolates in Immunodeficient Mice

Bacteroides are potential candidates  for next-generation probiotics (NGPs), which require preclinical safety and efficacy evaluations to ensure their rational use. This study aimed to verify the safety of two Bacteroides strains isolated from human fecal samples, Bacteroides dorei CK16 (B. dorei CK16) and Bacteroides vulgatus CK29 (B. vulgatus CK29), using genomic analysis and in vivo experiments. Whole-genome sequencing analysis of B. dorei CK16 revealed a predicted 4,898 protein-coding sequences (CDS), about 5.5 Mb of total genome length with a G + C content of 42.08%, and B. vulgatus CK29 revealed a predicted 4,610 CDS, about 5.3 Mb of total genome length with a G + C content of 42.56%. Moreover, the genome demonstrated the absence of virulence factors, and insertion sequences related to clinically relevant strains in either strain. A 42-day in vivo experiment was conducted on BALB/c and BALB/c nude mice, with each mouse receiving a daily dose of 1 × 10⁸ colony forming units (CFU) /mL of B. dorei CK16 or B. vulgatus CK29. No significant in vivo pathogenic characteristics were observed based on body weight, organ index, hematological, serum biochemical, or histological analyses, particularly in nude mice. Therefore, the initial safety assessment of the two novel Bacteroides strains exhibited no notable adverse effects in both immunocompetent and immunodeficient mice models.