Synbiotic

And then the idea is then they become beneficial because there's higher quantities of those organisms.

Synbiotics, which kind of is a logical trajectory from probiotics and prebiotics, is basically you do a combination of a prebiotic.

such as a beta-glucan fructooligosaccharide or one of those.

You combine that with, say, lactobacillus, which can use that as a substrate, and you feed them together with the idea that now you're setting up the table for lactobacillus to be enriched even more than it would have been because you're feeding it a substrate that it is already well attuned to.