Dietary Supplementation with Bacillus licheniformis and Bacillus subtilis Modulates Immunity, Serum Metabolome, and Intestinal Homeostasis in Cats

In recent years, as the number of cats has increased, the intestinal health of cats has receied increasing amounts of attention. Probiotics have positive effects on maintaining intestinal homeostasis. This study was conducted to investigate the effects of probiotics <i>Bacillus licheniformis</i> (<i>B. licheniformis</i>) and <i>Bacillus subtilis</i> (<i>B. subtilis</i>) on cat immunity, inflammation, antioxidants, intestinal barrier and microbiota, and serum metabolites. Thirty-six cats (over one year old, 3.48 ± 0.71 kg) were randomly divided into 3 groups and fed with a basal diet (CON group), a basal diet + <i>B. licheniformis</i> (BL group), and a basal diet + <i>B. subtilis</i> (BS Group). The experiment lasted 35 days. Fecal scoring indicates that <i>B. licheniformis</i> and <i>B. subtilis</i> can improve fecal scores. Serum analysis indicated that the addition of both substances increased levels of IgA, IgM, T-AOC, and SOD, while reducing levels of the pro-inflammatory factor TNF-α. Moreover, 16S rRNA gene sequencing revealed that <i>B. licheniformis</i> and <i>B. subtilis</i> altered the fecal microbiota composition, characterized by the elevated abundance of <i>Bacillus</i>. Adding <i>B. licheniformis</i> to the diet increased the level of <i>Faecalibacterium</i> and decreased the level of <i>Mogibacterium</i>. Serum metabolomics revealed that levels of L-Glycine and Sn-Glycero-3-phosphocholine exhibited marked elevation in both the BL and BS groups, respectively. Furthermore, the study demonstrated that differential metabolites in the BL group were mainly enriched in amino acid metabolism pathways, while those in the BS group were chiefly concentrated in lipid metabolism pathways. However, this study acknowledges the limitations of its exclusive use of Ragdoll cats and its 35-day intervention period. It highlights the need for future research involving diverse breeds and longer durations. Overall, the data highlight <i>B. licheniformis</i> and <i>B. subtilis</i> as cat nutritional supplements that improve immunity and maintain intestinal health.