The observed responses of these phyla to AI-2 could have to do with the level of expression of this chemical signal by these different species, the researchers noted: only 17 percent of Bacteroidetes are predicted to synthesize a version of AI-2 compared to 83 percent of Firmicutes species. The presence of AI-2 appeared to favor the growth of Firmicutes species known to contribute to the health of the mammalian gut.