A comparison of the phenylpyruvic acid reaction and the urease test in the differentiation of Proteus from other enteric organisms

that Proteus strains could be differentiated from other enteric organisms by their action on phenylalanine (PA). Whereas the former rapidly produced large quantities of phenylpyruvic acid (PPA), demonstrable by a color reaction with ferric ions, the latter produced only small quantities of PPA or none at all. A rapid test was described that seemed to be highly specific for Proteus. At that time no simple method was available for testing urease activity, and no comparison of the two reactions was made. In the last few years, however, several good methods for the study of urea decomposition have become avail-able. Thus Rustigian and Stuart (1941), Ferguson and Hook (1943), Christensen (1946), and Elek (1948) have described different methods. Stuart, van Stratum, and Rustigian (1945) found the method of Rustigian and Stuart very specific. Out of 600 Proteus strains only 5 gave weak or negative reactions, whereas 1,000 cultures of other enteric organisms failed to give posi-tive reactions within 48 hours. Christensen claimed that this medium would only allow the growth of or-ganisms capable of utilizing urea as a sole source of nitrogen, and that organisms capable of hydrolyzing urea, but incapable of utilizing ammonia as a source of nitrogen, might escape detection. Consequently he devised a medium that is richer in nutritive substances (peptone and glucose). On this medium Proteus strains give a much more rapid and a much stronger reaction than other or-ganisms, but paracolon, Aerobacter, and intermediate types might produce a weak positive reaction within 24 hours and a strong reaction after 3 to 5 days. Thus both media are well suited for the differentiation of Proteus from other organisms. That of Rustigian and Stuart seems to be more specific, whereas that of Christensen is better suited for the detection of weak urease activity. The results of a comparison of these reactions were reported by Cook (1948)