Evaluation of the rapid hippurate hydrolysis test with enterococcal group D streptococci.

The rapid hippurate hydrolysis test was evaluated with the conventional test, using 17 group A streptococci, 9 non-enterococcal group D streptococci, 108 enterococcal group D streptococci, and 2 strains of Listeria monocytogenes. There was complete correlation between the rapid and conventional tests with all organisms except enterococcal group D. The rapid hippurate hydrolysis method was more sensitive with the enterococci; 95.4% were positive with the rapid method, and 9.3% were positive with the conventional method. Thin-layer chromatography (TLC) was performed on all isolates to determine if the end product of hydrolysis, glycine, was indeed present. The TLC results were in agreement with the rapid and conventional methods for group A streptococci, nonenterococcal group D streptococci, and L. monocytogenes. TLC results were in total agreement with the rapid hippurate hydrolysis test for the enterococcal group D isolates, thus verifying the accuracy of this more sensitive test. Trace amounts of glycine were found in the substrate, indicating the need for including an uninoculated substrate control as well as stock strains of group A and B beta-hemolytic streptococci (negative and positive controls, respectively) each time the rapid hippurate hydrolysis test is performed.