Additive effect of mixtures of multiple strains combining the effects of single strains on human gut microbiota <i>in vitro</i>

Probiotics exert their beneficial effects by improving the intestinal environment. Heat-inactivated probiotics may show similar effects. However, whether multi-strain mixtures (MSM) are better than single strains, irrespective of whether the bacteria are alive or dead, is unknown. In this study, we examined the gut-improving efficacy of an MSM consisting of four dead bacterial strains using an <i>in vitro</i> model that simulates the human gut environment. In the <i>in vitro</i> model, human feces were inoculated with a single-strain or MSM, and the microbial composition and fermentation products were assessed. The 16S rRNA gene sequences showed that the MSM tended to change the microbial community structure more than single strains. Furthermore, the results of a microbial diversity analysis showed that despite differences among single strains, the effect of the MSM increased with the abundance of any single strain. A similar trend was observed for fermentation products. These results suggested that the MSM made from dead bacteria exerted additive effects that may provide new health benefits to more people.