Which of the following organisms is nonmotile at 37 degrees C?

Shigella dysenteriae is indeed a nonmotile organism at 37 degrees Celsius. This characteristic helps differentiate it from other closely related bacteria. Shigella species, including Shigella dysenteriae, do not possess flagella and therefore do not exhibit motility, which is a critical feature for identifying and classifying these bacteria in clinical microbiology.

In contrast, the other organisms listed possess motility mechanisms. For instance, Escherichia coli and Salmonella typhimurium both have flagella that enable them to move actively in liquid environments. Proteus vulgaris is also known for its distinctive swarming motility due to its numerous flagella, particularly when grown on solid media. These motile characteristics can aid in the differentiation and identification of these bacteria in a laboratory setting, making Shigella dysenteriae unique in that it remains nonmotile at the specified temperature.