Rhizobium species colonize legume roots forming nodule structures. They are slow-growing in contrast to "Rhizobium" species, which are considered fast-growing rhizobia. In addition, soybean uses isoflavones to stimulate soil-microbe rhizobium to form nitrogen-fixing root nodules. Tagasaste is considered to be a promiscuous legume, compatible with cowpea and Tagasaste 1502 Rhizobium. An example is "Rhizobium" bacteria growing in nodules on the roots of legumes. Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Acacia auriculiformis can fix nitrogen after nodulating with a range of Rhizobium and Bradyrhizobium strains. "Agrobacterium" has been recognized as being related to the "Rhizobium". The enzyme occurs in "Rhizobium" bacteria. Often, Rhizobium infected cells have only small vacuoles. To prevent cheating, these plants seem to be able to punish the rhizobium bacteria. Bacteria genera "Rhizobium" and "Mesorhizobium" work to fix nitrogen into a usable form. The curling begins with the very tip of the root hair curling around the "Rhizobium". Prior to sowing, the seeds are subject to Rhizobium culture. The most commonly applied rhizobacteria are "Rhizobium" and closely related genera.