Which of the following is not urease positive?

• A. Proteus mirabilis
• B. Helicobacter pylori
• C. Yersinia enterocolitica
• D. Salmonella

Answer: (D)

Urease breaks down urea into ammonia and carbon dioxide, raising the pH around the organism. In a urease test, organisms that produce urease rapidly hydrolyze urea, causing an alkaline shift that is detected by a color change in the medium’s pH indicator.

Proteus mirabilis is a classic urease producer, so its urease activity quickly triggers an alkaline change. Helicobacter pylori also uses urease to neutralize stomach acid, making it urease positive as well. Yersinia enterocolitica typically shows urease activity, so it can give a positive result in urease testing. Salmonella, however, generally does not produce urease, so the urease test stays negative for it. Therefore, the organism not urease positive is Salmonella.