What allows grass eaters to digest cellulose?
No there is something else missing besides bacteria

humans often already have the right bacteria but they don't digest most of the cellulose they ingest because digesting cellulose with bacteria is SLOW. Too slow to keep up with normal mammalian digestion. You need a specialized gut that  either has a large complex stomach to keep recycling the cellulose in the system (foregut fermenters) or has large storage chambers to keep the cellulose a long time and allow it to be digested slowly (hindgut fermentation). 

humans have neither. To make matters worse Humans have huge caloric needs compared to other animals of the same size. Brains are calorie hogs and our brains are so large they consume 1/4 of all our calories. That is 3 pounds of our mass consumes a quarter of all the calories we eat. honestly without the advent of cooking (which drastically improves the caloric availability of food) human brains could never have reached the size they did.  

Further reading

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