Current issues pertaining to the Nocardia species

Because the number of recognized Nocardia species is continuing to increase, members of the genus Nocardia are becoming increasingly difficult to identify using phenotypic criteria. Molecular techniques, such as restriction endonuclease analysis and 16S rRNA gene sequencing, have become increasingly useful methodologies for the definitive identification of clinical isolates in this genus. While molecular methods have been useful for rapid and accurate identification, their use has complicated nocardial taxonomy by revealing new, clinically relevant species that are genetically distinct from known species but which may have few distinguishing phenotypic characteristics. We present here an overview of some taxonomic issues concerning Nocardia species and describe molecular techniques that are useful in the identification of the majority of clinically relevant species.