Citrobacter is a genus of Gram-negative coliform bacteria in the family Enterobacteriaceae. The species C. amalonaticus, C. koseri, and C. freundii can use citrate as a sole carbon source. Citrobacter species are differentiated by their ability to convert tryptophan to indole (C. koseri is the only citrobacter to be commonly indole-positive), ferment lactose (C. koseri is a lactose fermentor), and use malonate. Citrobacter shows the ability to accumulate uranium by building phosphate complexes.