The differentiation of Trichophyton rubrum and T. mentagrophytes by use of Christensen's urea broth

A comparison of the urease activity of 56 strains of T. rubrum and 64 strains of T. mentagrophytes on Christensen's urea medium with and without agar incubated 7 days at 28 °C showed that characteristic and consistent results are produced on this medium without agar. In the fluid medium T. mentagrophytes caused a rise in pH, resulting in a maximum change of the phenol red, and T. rubrum produced no change. The depressing effect of agar on the hydrolysis of urea was demonstrated. On the agar medium 9 (18.8%) of 48 strains of T. mentagrophytes and 3 (6.2%) of 48 strains of T. rubrum produced doubtful results. It is important that pure cultures be used for the test.