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 species are classified based on their capacity to convert tryptophan into indole, ferment lactose, and utilize malonate, characteristics shared by C. koseri.