SRD:Greater Magic Fang

Magic fang gives one natural weapon of the subject a +1 per four caster levels (maximum +5) enhancement bonus on attack and damage rolls. The spell can affect a slam attack, fist, bite, or other natural weapon. (The spell does not change an unarmed strike’s damage from nonlethal damage to lethal damage.)

Alternatively, you may imbue all of the creature’s natural weapons with a +1 enhancement bonus (regardless of your caster level).

Greater magic fang can be made permanent with a permanency spell.