An organism isolated from a stool culture is a Gram-negative rod, urease and PD negative, and shows certain results in TSIA. Which organism does this indicate?

The organism described in the question is a Gram-negative rod that is urease and phenylalanine deaminase (PD) negative, along with specific results in Triple Sugar Iron Agar (TSIA). This combination of characteristics strongly suggests Shigella species as the identifying organism.

Shigella species are non-lactose fermenting Gram-negative rods that do not produce urease or phenylalanine deaminase. This aligns with the positive identification criteria for Shigella, as these bacteria are typically associated with gastrointestinal infections and are unable to ferment carbohydrates in a manner that produces gas or acid, reflected in TSIA results. The absence of urease is crucial since other Gram-negative organisms, like Proteus, would typically yield a positive urease reaction.

In certain TSIA tests, Shigella tends to produce a yellow slant and butt due to glucose fermentation, without the production of gas or hydrogen sulfide, which further distinguishes it from other Enterobacteriaceae. Given these specific traits and their relation to gastrointestinal disease patterns, the identification of the organism as Shigella provides a clear link to the clinical implications and microbiological characteristics described.