There are three dietary components to a healthy gut; probiotics, prebiotics (i.e. fermentable fiber), and polyphenols. Of the three, the probiotics have the least impact, whereas the prebiotics and polyphenols can actually help change the composition of the gut microbiome to a less inflammatory state.
Recent years have seen an increasing focus on the human microbiome, the ecology of bacteria that live within us. In particular, there has been an intense focus on how our gut fauna influence health conditions, namely obesity.