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.
Differentiation of Citrobacter species occurs through their ability to convert tryptophan into indole, with C. koseri being the only notable exception; they also ferment lactose and use malonate as a carbon source.