"I take it you are not advocating microbial supplements like soil based organisms?"

Ah, no. But I'm curious to know where you think our gut microorganisms originate?

Of course they come from soil microcosms, from foods we prepare and ingest. Any gram soil sample contains millions of isolates (the concept of species among microbes is highly contested)

An absence of exposure to nonpathogen microoganisms in our sterile homes, by consumption of highly processed foods, and puritanical use of cleaning products as been identified as one of many reasons for declining immunity among Western nations.

The best means of encouraging retention and growth of a richly varied gut microbial community is by a change in diet towards reduced dependence on prepared foods, and deliberate consumption of wholesome and varied food sources (slower release proteins, carbs, and healthy fats) and yes, the use of certain foods that also serve as concentrated source of the most common types of gut bacteria.