How do bacteria help cows digest their food?

Bacteria produce various enzymes unique to them, depending on what kind of bacteria they are and what substance of the plant material they target, which break down the plant matter that the cow has consumed. For example, the cellulose-digesting bacteria produce cellulase, an enzyme used to break the chemical bonds which make up cellulose in order to obtain energy in the form of ATP, and produce volatile fatty acids, the "big three" including proprionate, acetate and butyrate. The microbes, once they've lived out their lifespan, will move further along the digestive tract of the cow and get digested by the cow herself and provide a good source of microbial protein.