Which Nocardia Species Is It, Anyway?

Abstract & Commentary

Synopsis: Molecular techniques provide a more rapid and accurate method of Nocardia species identification than do classical microbiologic methods. A surprise: of 94 isolates, the most commonly identified species was Nocardia cyriacigeorgica and none were Nocardia asteroides.

Source: Joann L. Cloud, et al. Evaluation of Partial 16S Ribosomal DNA Sequencing for Identification of Nocardia Species by Using the MicroSeq 500 System with an Expanded Database. J. Clin. Microbiol. 2004;42:578-584.

The 16S ribosomal DNA sequencing is very useful for speciating many microbial pathogens. Workers in Utah, the NIH, and Munster, Germany, combined forces to determine the species of 94 non-repetitive Nocardia isolates. A 16S ribosomal DNA sequencing system called MicroSeq 500 was compared to conventional biochemical and susceptibility speciation routines. The "gold standard" was an NIH speciation routine that has been previously described (J Clin Microbiol. 2000;38:158-164). The species that are routinely identified by the MicroSeq 500 are shown in the Table below.

There are 31 validly named species.of Nocardia Results of this study showed the molecular method was more discriminative than the conventional method and identified 10 species. Previously identified human pathogens like Nocodia africana, Nocodia paucivorans, and Nocodia pseudobrasiliensis were not found in this study.

Readers may be most surprised by the species of the most commonly isolated Nocardia, Nocardia cyriacigeorgica. The conventional analysis would be Nocardia asteroides. None of the isolates in this study were identical to the N. asteroides type strains. The second most common groups were Nocardia farcinica (14) and Nocardia nova (16) followed by N. africana (11), and Nocardia veterana (10). There were 8 isolates identified as N. asteroides drug pattern IV and 8 as Nocardia abscessus. The MicroSeq identified 14 isolates as Nocardia but gave no definitive species.

Comment by Joseph J. John, Jr, MD

Nocardia are becoming an ever more important cause of opportunistic infection. Conventional identification has always been fraught with problems, as shown in this current study by Cloud and colleagues. Clinicians remember the species they grew up with, N. asteroides, but they will need to expand their minds consistent with the expansion of species within the genus.

As this study shows, we need to expand the identification of the pathogenic species we are seeing clinically. More N. asteroides cases will become N. cyriacigeorgia. The other important names needing our familiarity include veterna, nova, and farcinica.

It is hard to say how soon standard clinical micobiological laboratories will choose to use the MicroSeq system. One of the study participants in the Cloud study was the ARUP Institute, a lab that many of our university clinical microbiology now uses. Clinicians will have to decide based on patient characteristics and the need for expanded epidemiologic information how often to ask for speciation. MicroSeq speciation takes only1-3 days, compared to the conventional time of 2-3 weeks. In fast moving specialities like transplantation, patient care may mandate the more rapid and clearly more definitive method of DNA sequencing and correlation of bactericidal activity with the MICs of the organisms—all of which were < 2 mg/mL.

Joseph F. John, Jr., MD, Chief, Medical Subspecialty Services, Ralph H. Johnson Veterans Administration Medical Center; Professor of Medicine, Medical University of South Carolina, Charleston, SC, and Co-Editor of Infectious Disease Alert.