Coloured scanning electron micrograph (SEM) of Bradyrhizobium is a genus of Gram-negative, soil bacteria in the family Bradyrhizobiaceae. Many of the species of this genus fix nitrogen. It forms symbiotic relationship with modulated plant root systems (legumes). The bacteria contain a nitrogenase enzyme system that reduces atmospheric nitrogen and produces nitrogen compounds that the plant can use for growth. The plants provide organic compounds for the bacteria to use. Nitrogen fixation is an important part of the nitrogen cycle on this planet. Plants cannot use atmospheric nitrogen they must use nitrogen compounds such as nitrates and nitrites. Magnification: x5, 000 when shortest axis printed at 25 millimetres.