Which of the following bacteria is oxidase-positive and often associated with gastroenteritis in seawater exposure?

• A. Escherichia coli
• B. Shigella
• C. Vibrio species
• D. Salmonella

Answer: (C)

The key idea is the oxidase test result and the typical ecological association. Vibrio species are oxidase-positive, meaning they have the enzyme cytochrome c oxidase and give a positive oxidase test. This helps separate them from the Enterobacteriaceae listed here—Escherichia coli, Shigella, and Salmonella—which are oxidase-negative. Clinically, Vibrio is well known for causing gastroenteritis after seawater exposure or consumption of raw seafood, with organisms such as Vibrio parahaemolyticus and Vibrio vulnificus being classic culprits. So when you see a gastroenteritis case linked to seawater exposure, oxidase-positive testing strongly points toward Vibrio rather than the other options.