When we eat more and various forms of soluble fibers, we get more beneficial, non-caloric substances our bodies have always needed, as these then grow more such bacteria in our gut.
This study did not use bacteria, instead, it used the produced substances to see what they did to help us. Generally, we should have these bacteria already in our gut, unless we wiped them out with antibiotics at some time.
Different soluble fibers provide different beneficial substances. Examples of various soluble fibers capable of these separate benefits include resistant starch, inulin, alginate, guar gum, psyllium, FOS, polydextrose, GOS, and soy fiber.