Here they do a dilution to extinction experiment where they purposefully screen for wells that do *not* grow on solid media. What do they find? A huge enrichment in undercultivated microbes: Acidobacteria, Verrucomicrobia, Armatimondaetes!
Really simple, but seems really effective!
https://www.frontiersin.org/articles/10.3389/fmicb.2023.1194466/full