Subject: What's the plausibility of introducing cellulose digestion into humans ?

I would also like to know which enzyme/bacteria aids animals in the break-down of cellulose into simpler sugars. Could these live in our stomachs? Would our immune systems attack them? If so could we stop the attack? Could they be cheaply mass produced? Could we use them outside of the body to make things like grass edible (regardless of taste)? I am interested in finding out whether or not we have any way to utilize an untouched food source. Would it be possible, through genetic engineering or an other means, to allow humans to break down even some cellulose ?

Re: What's the plausibility of introducing cellulose digestion into humans ?