Christensen later formulated a medium that would allow growth of other members of the Enterobacteriaceae that cannot utilize the by-product of urea hydrolysis (ammonia) as a sole nitrogen source for growth (2). This medium also has a decreased buffering capacity allowing for the detection of smaller quantities of alkali produced from the degradation of urea. Thus organisms with a slight or delayed urease activity can be detected with Christensen’s urea agar.