Protective Effects of Bacillus subtilis HH2 against Oral Enterotoxigenic Escherichia coli in Beagles

Simple Summary Enterotoxigenic Escherichia coli (ETEC) is an important pathogen that causes diarrhea in both humans and animals, thereby posing a serious threat to public health and animal agriculture. Bacillus subtilis, a probiotic, offers a novel approach to reduce the need for antibiotics and plays a crucial role in treating various intestinal diseases. We previously isolated a strain of B. subtilis HH2 from giant panda feces, which has shown multiple beneficial functions in vitro and in vivo. However, studies on the protective effect of B. subtilis on companion animals with orally administered ETEC have not been reported. Therefore, we explored the effects of B. subtilis HH2 on the fecal microbiota, intestinal barrier integrity, and non-specific immunity in beagles challenged with ETEC. Experimental results showed that B. subtilis HH2 could alleviate diarrhea caused by ETEC, improve non-specific immunity and intestinal barrier integrity, and modulate gut microbiota. Notably, more indicators are needed to determine its protective effect on beagles in future studies. This study evaluated the protective effect of Bacillus subtilis HH2 on beagles orally challenged with enterotoxigenic Escherichia coli (ETEC). We assessed the physiological parameters and the severity of diarrhea, as well as the changes in three serum immunoglobulins (IgG, IgA, and IgM), plasma diamine oxidase (DAO), D-lactate (D-LA), and the fecal microbiome. Feeding B. subtilis HH2 significantly reduced the severity of diarrhea after the ETEC challenge (p < 0.05) and increased serum levels of IgG, IgA, and IgM (p < 0.01). B. subtilis HH2 administration also reduced serum levels of DAO at 48 h after the ETEC challenge (p < 0.05), but no significant changes were observed in D-LA (p > 0.05). Oral ETEC challenge significantly reduced the richness and diversity of gut microbiota in beagles not pre-fed with B. subtilis HH2 (p < 0.05), while B. subtilis HH2 feeding and oral ETEC challenge significantly altered the gut microbiota structure of beagles (p < 0.01). Moreover, 14 days of B. subtilis HH2 feeding reduced the relative abundance of Deinococcus-Thermus in feces. This study reveals that B. subtilis HH2 alleviates diarrhea caused by ETEC, enhances non-specific immunity, reduces ETEC-induced damage to the intestinal mucosa, and regulates gut microbiota composition.