Leguminous plant and rhizobium sp is known as

Rhizobium is a genus of gram-negative, motile bacteria whose members are most notable for their ability to establish a symbiotic relationship with leguminous plants, such as peas, soybeans, and alfalfa.

In the assay set up immediately after incubation there will still be some bacteria present who are ampicillin sensitive that have survived incubation however in the assay made after an extended period there will only be bacteria that are completely ampicillin resistant as all sensitive bacteria will have died.