cat·a·lyst
noun
1.Chemistry . a substance that causes or accelerates a chemical reaction without itself being affected.
2.something that causes activity between two or more persons or forces without itself being affected.
3.a person or thing that precipitates an event or change: His imprisonment by the government served as the catalyst that helped transform social unrest into revolution.
4.a person whose talk, enthusiasm, or energy causes others to be more friendly, enthusiastic, or energetic.

rhizobium
Any of various nitrogen-fixing bacteria of the genus Rhizobium that form nodules on the roots of leguminous plants, such as clover and beans