Characterization of human clinical isolates of Dietzia species previously misidentified as Rhodococcus equi.

In this study, 16 human clinical isolates of Dietzia species previously misidentified as Rhodococcus equi were evaluated using phenotypic methods, including traditional and commercial (API Coryne) biochemical tests, antimicrobial susceptibility testing, and 16S rRNA gene and gyrB gene sequencing. Positive results for both the hydrolysis of adenine and Christie-Atkins-Munch-Petersen (CAMP) reaction allowed for differentiation between the Dietzia isolates and the type strain of Rhodococcus equi; however, traditional and commercial phenotypic profiles could not be used to reliably identify Dietzia species. The analysis of 16S rRNA gene and gyrB gene sequences could discriminate all Dietzia strains from the type strain of R. equi. Most Dietzia species had distinct 16S rRNA gene and gyrB gene sequences; however, the 16S rRNA gene sequences of the type strains of D. schimae and D. cercidiphylli were identical to D. maris and D. natronolimnaea, respectively. Based on comparative sequence analysis, five clinical isolates clustered with D. maris/D. schimae and nine with D. natronolimnaea/D. cercidiphylli. The two remaining isolates were found to be most closely related to the D. cinnamea/D. papillomatosis clade. Even though molecular analyses were not sufficiently discriminative to accurately identify all Dietzia species, the method was able to reliably identify isolates that were previously misidentified by phenotypic methods to the genus level.