The friendly bacteria produces many byproduct acids by fermenting fiber/carbs. The release of these acids helps to push the bowels out/ give you the sudden urge to poop. The same effect when someone gets food poisining the body produces excess HCL to try to kill the bad bacteria. The excess Hydrocholric "acid" causes diarreah/fushes the bowels out. All the acids that the flora produces creates an acidic environment in the colon. Feces contain 30-60% friendly bacteria.
When i lost my friendly bacteria due to anti-bacteria substances i became completely constipated.