The Nitrobacteraceae are a family of gram-negative, aerobic bacteria. They include plant-associated bacteria such as Bradyrhizobium, a genus of rhizobia associated with some legumes. It also contains animal-associated bacteria such as Afipia felis, formerly thought to cause cat-scratch disease. Others are free-living, such as Rhodopseudomonas, a purple bacterium found in marine water and soils. The strain Rhodopseudomonas palustris DX-1 can generate an electric current with no hydrogen production, a trait being explored in the development of the microbial fuel cell. The genus Afipia has also been found in the atmosphere, where it uses methylsulfonylmethane as a carbon source.