If you will manage to teach your dog to be careful while he uses his teeth and opens his mouth, this won't make him unable to guard and defend, but the dog will realize his power and will know how to control it.
Begin the study from early years. Anyway you will notice at once that teeth of your puppy are so sharp that you may even accidentally cut your hand. A young puppy likes to taste everything he finds. If a puppy tries to bite your hand or leg, you must say: "Leave, don't bite!" and strike his nose abruptly. The point is that you must strike abruptly! Some owners don't dare to hit a puppy and only make a slight touch. This encourages the puppy to continue biting master's hand. Being shy is of no use and stimulates aggressiveness in him. One good blow to make him feel it would be enough to stop biting. Puppies very quickly begin holding in respect those who modulate their behavior. You should repeat the punishment if the puppy occurs to have a short memory. After your puppy understands the behavior you are waiting from him you'll be able to correct him by words: "Leave, don't bite".
Use the command every time the puppy starts using his teeth - bites someone, chews a chair leg, twitches a sofa, chews slippers or snatches the elbow-rest in a car. The owner who favors his puppy to bite and be rude during game stimulates bad habits. Such puppy may grow up hard to be controled, may become rude and aggressive, and dangerous to live with in one house. A dog raised without any control, becomes so disobedient that ultimately the owner has to train his dog by himself or give it to a professional trainer. In both cases the dog will resist the attempt of trainer to correct his behavior and one will have to instill good manners by means of power.
If it happens so that a Berner Sennenhund passes through hands of several owners, he may go so out of control that at last the owner's patience gets exhausted. Therefore if the owner starts teaching his puppy the norms of behavior while the pup is small, everybody will be able in future to control him and the dog will try to please his master.
It's useful from time to time to vary this lesson. For example, if you use a toy for play and a puppy gets so excited that starts biting, say to him gently and calmly: "Easy!" If he knows the lesson with food well, he will settle down and will play carefully.