Human gut associated <i>Bacteroides</i> and <i>Akkermansia</i> bacteria exhibit immunostimulatory activity in the silkworm muscle contraction assay

The immunoregulatory activity of human gut bacteria has attracted attention in recent years. To assess the innate immune-stimulatory activity of various samples in vivo efficiently, we previously introduced a silkwormbased assay as a novel alternative method. The method has been used for over a decade to screen for substances with potential physiological activity. In this study, we prepared heat-killed cells of four strains of human gut bacteria (Bacteroides ovatus, B. thetaiotaomicron, B. uniformis, and Akkermansia muciniphila) and assessed their innate immune-stimulatory activity within the silkworm model. Our findings indicate that the sample from either B. ovatus or B. thetaiotaomicron has immunostimulatory activity in the silkworm, in contrast to B. uniformis and A. muciniphila. Moreover, a pathogenicity assessment using the silkworm infection model was conducted to determine the safety of these bacterial strains for human consumption when considered as food ingredients. None of the four gut bacterial strains exhibited pathogenic effects in silkworms, with Pseudomonas aeruginosa serving as a positive control of the pathogenicity test. These results suggest that the silkworm-based assay can distinguish between the immunostimulatory effects of different human gut microbes and may enhance the safety evaluation of microbial ingredients.