Clinical Laboratory Science Practice Exam 2026 – Your All-in-One Guide to Exam Success!

The main concept is using a biochemical test that reliably differentiates Citrobacter from Salmonella: lysine decarboxylase activity. Citrobacter species are typically positive for lysine decarboxylase, meaning they can convert lysine to cadaverine in a decarboxylation medium, which produces an alkaline environment and a positive result. Salmonella, on the other hand, is generally negative for this test, so it does not produce the same alkaline end products. This clear contrast makes the lysine decarboxylase test a strong discriminator between these two genera.

Other common tests don’t separate them as reliably. Hydrogen sulfide production can occur in both groups depending on the strain and media conditions, so it isn’t definitive. Indole production can vary among Citrobacter species and isn’t consistently distinguishing from Salmonella. Urease activity is not a reliable differentiator for these two organisms either, as their urease patterns do not provide a consistent separation. Therefore, the lysine decarboxylase result offers the most consistent single test to tell Citrobacter apart from Salmonella.