Urea Decomposition as a Means of Differentiating Proteus and Paracolon Cultures from Each Other and from Salmonella and Shigella Types.

Bacterial decomposition of urea has been especially useful in distinguishing between members of the Proteus group and other fecal organisms encountered in stool examinations. Originally, the reactions of these organisms on urea were determined by the use of sterile urine as a culture medium, and only recently have media of known composition been employed (Rustigian and Stuart, 1941). Of the media of known composition, those of Rustigian and Stuart and of Ferguson and Hook (1943) have been found suitable to differentiate between Proteus and other organisms believed to be urease-negative. In considering the formulae of the two media mentioned, it is obvious that the medium of Ferguson and Hook (urea, 2 per cent; phosphate buffer; NaCl, 0.5 per cent; and ethyl alcohol, 1.0 per cent) is of such a nature that only an organism capable of utilizing urea as a sole source of nitrogen would grow. The possibility therefore arises that this medium would not detect organisms which could hydrolyze urea but which could not utilize the liberated ammonia as a source of nitrogen. In such a case, no growth would occur, and the organism would be considered urease-negative. The medium of Rustigian and Stuart (urea, 2 per cent; yeast extract, 0.01 per cent; and phosphate buffer), because of the extremely small amount of nutritive material present, also is subject to the same theoretical limitations, particularly in view of the fact that an organism incapable of utilizing ammonia is probably forced to use the yeast extract not only as a nitrogen source but also as a carbon source. If these materials are exhausted before the organism shows appreciable growth, its ability to hydrolyze urea cannot be adequately determined. The high buffer capacity of the medium would also mask slight urease activity if such an organism were capable of initial growth in the medium. These two media, however, as stated by the authors, were designed for the detection of Proteus organisms, and for this purpose they are well suited. It was thought possible, however, that if a medium were devised which eliminated the necessity of an organism's utilizing ammonia as a sole source of nitrogen, other organisms of the fecal group could be detected which would hydrolyze urea.