|MadSci Network: Microbiology|
In fact, the fiber we eat and our intestinal bacteria haven't a great deal to do with each other. The fiber performs an important water-retaining function, to keep us "regular." Some animals can digest fiber with the help of bacteria and other microorganisms, but humans essentially don't have that kind of microbes in their digestive system.
If your home is attached to a city sewer system, fiber such as toilet paper is already going to the sewage treatment plant. If you have a "disposal" (sometimes called a garbage grinder), you are probably sending other kinds of fiber, mostly plant waste, to the sewage treatment plant, along with lint from the laundry. These probably don't do much in the sewer on the way to the treatment plant. They settle out reasonably well in the first stage of sewage treatment and become part of "primary sludge." The sludge is processed so as not to be too smelly (and may yield some useful biogas); it may then be used as fertilizer, directly or after composting.
If your home has a septic tank, the fiber will mostly settle with sludge at the bottom of the tank. It is important not to let the tank get too full of sludge, or the sludge may carry over to the soil field where the treated water is supposed to seep into the ground. If this soil field gets clogged and the wastewater can't soak in, the wastewater will rise to the surface of the ground or back up into the house and make a mess. For these reasons, people whose homes have septic tanks are extra careful of the kinds of toilet paper they use, and they usually avoid having a garbage grinder that will put extra fiber into the tank.
So, whether your home is attached to a sewer system or to a septic tank, there is not much point in sending extra fiber to the bacteria. Fiber is nutritional for cows and other ruminants and for horses and rabbits (which digest it in another way). There are products sold that are said to digest the sludge (including fiber) in septic tanks by bacterial action, but reports from the US Environmental Protection Agency say these don't do much good. Fiber may be as soft as cotton or as scratchy as a rough rope, but it's pretty tough stuff after all.
Try the links in the MadSci Library for more information on Microbiology.