Comparison of selective media for the isolation of Rhodococcus equi and description of a new selective plating medium.

In order to improve the isolation rate of Rhodococcus equi from animals and soil, the efficacy of four previously described selective media (CAZ-NB, M3T, NANAT and TINSDALE) and that of four other media (NC, PNP, TCP and TVP) composed by us was compared and evaluated. Two selective plating media proved to be the best for the isolation of R. equi from contaminated samples. One of them was CAZ-NB containing ceftazidime, novobiocin and cycloheximide, while the other was the newly composed TCP containing trimethoprim, cefoperazone, polymyxin B, cycloheximide and potassium tellurite as selective components. These two media allowed the growth of at least 62-72% of R. equi present in the artificially contaminated samples, and the inhibition of unwanted contaminant bacteria and fungi was satisfactory with both media. TCP medium proved to be superior to CAZ-NB since the colony morphology of R. equi was much more characteristic (shiny, smooth, black colonies 3-5 mm in diameter) on it, and it inhibited the unwanted contaminant bacterial and fungal flora more effectively, especially in the case of faecal and soil samples. Therefore, TCP is recommended as a new, highly selective plating medium for the isolation of R. equi from contaminated samples.