A kefir grain resembles a piece of cauliflower, except that it has a rubbery texture; see the picture on Dom's kefir in-site. This outer membrane is a soluble polysaccharide complex called kefiran. The probiotic bacteria and yeast synthesize this substance, and are enclosed within it. What amazes me is how a colony of supposedly unintelligent single-celled organisms knows enough to act in their mutual self-interest and make a "cell wall" around the entire colony. Not only that, the wall adapts by growing with the colony, as in this picture on Dom's kefir in-site. It even heals itself when breached, such as when a "mother" grain is pulled apart into two "baby" grains, as in this animation.