What is the primary substance used by bacteria in the cecum for digestion?

The primary substance used by bacteria in the cecum for digestion is roughage, which consists mainly of fiber from plant materials. In many animals, particularly herbivores and omnivores, the cecum is a crucial site for the fermentation and breakdown of complex carbohydrates, primarily found in roughage.

Bacteria in the cecum utilize roughage as a substrate for fermentation, breaking it down into simpler compounds that can be absorbed and used by the host animal. This process not only aids in digestion but also contributes to the production of volatile fatty acids, which serve as an essential energy source for the host.

In contrast, starch, protein, and water play different roles in digestion. Starch is primarily digested in the mouth and small intestine, protein is broken down in the stomach and small intestine, and while water is essential for various bodily functions, it does not comprise a substrate for bacterial fermentation. Thus, roughage is the substance that directly supports the bacterial activity in the cecum, making it the correct answer.