Which two tests are best to differentiate A. hydrophilia from P. shigelloides?

The selection of indole and lysine decarboxylase tests as the correct method for differentiating between Aeromonas hydrophila and Plesiomonas shigelloides is based on the unique biochemical characteristics of these two bacterial species.

Aeromonas hydrophila is typically indole-positive, meaning it can produce indole from tryptophan, which is a distinctive feature in the identification process. On the other hand, Plesiomonas shigelloides is usually indole-negative. Therefore, the indole test serves as a clear differentiator between these two organisms.

Additionally, the lysine decarboxylase test distinguishes the ability of some bacteria to decarboxylate lysine to produce cadaverine. A. hydrophila is typically lysine decarboxylase-positive, while P. shigelloides is usually negative. This additional biochemical trait provides further confirmation in differentiating between the two species.

Collectively, these tests exploit specific metabolic pathways characteristic to each organism, making them reliable for the differentiation of A. hydrophila and P. shigelloides in a clinical or laboratory setting.