This photograph depicts the results of a urease test, based on the process involving the hydrolysis of urea. The test was performed to help identify the Gram-negative, enteric bacterium, Yersinia enterocolitica. The hydrolysis of urea is confirmed by the formation of ammonia, and CO2, which alkalinizes the medium, and results in a pH shift. A hot-pink coloration, as seen in the tube at right, indicates a positive reaction, i.e., urea hydrolysis, and a yellow color, as in the tube on the left, indicates a negative reaction. Y. enterocolitica bacteria contain the urease gene cluster that enables them to hydrolyze urea, however, though this test may yield positive results, Y. enterocolitica is not the only organism that can perform this process, and further testing would be required in order to rule out the presence of other urease producing bacteria.