A lactose-negative gram-negative rod that tests positive for indole and urease is likely which organism?

The organism described as a lactose-negative gram-negative rod that tests positive for indole and urease is typically indicative of Morganella spp. This genus of bacteria is characterized by its biochemical reactions, particularly its ability to produce indole and urease, which are key identifiers in differentiating it from other gram-negative rods.

Morganella spp. are known to test negative for lactose fermentation, meaning they do not utilize lactose as a carbon source, which is a common feature among certain Enterobacteriaceae. The positive urease test suggests the ability of the organism to hydrolyze urea resulting in the production of ammonia and raising the pH of the medium, which is another trait of Morganella.

In comparison, while Edwardsiella and Ewingella may share some characteristics with Morganella, they do not typically test positive for both indole and urease together. Shigella species are generally lactose-negative but do not produce urease, making them inconsistent with the described characteristics. Therefore, the combination of lactose-negative status, positive indole production, and positive urease test strongly supports Morganella spp. as the correct answer.