Nocardia species identification is difficult due to a complex and rapidly changing taxonomy, the failure of 16S rRNA and cellular fatty acid analysis to discriminate many species, and the unreliability of biochemical testing. Here, Nocardia species identification was achieved through multilocus sequence analysis (MLSA) of gyrase B of the ␤ subunit of DNA topoisomerase (gyrB), 16S rRNA (16S), subunit A of SecA preprotein translocase (secA1), the 65-kDa heat shock protein (hsp65), and RNA polymerase (rpoB) applied to 190 clinical, 36 type, and 11 reference strains. Phylogenetic analysis resolved 30 sequence clusters with high (>85%) bootstrap support. Since most clusters contained a single type strain and the analysis corroborated current knowledge of Nocardia taxonomy, the sequence clusters were equated with species clusters and MLSA was deemed appropriate for species identification. By comparison, single-locus analysis was inadequate because it failed to resolve species clusters, partly due to the presence of foreign alleles in 22.1% of isolates. While MLSA identified the species of the majority (71.3%) of strains, it also identified clusters that may correspond to new species. The correlation of the identities by MLSA with those determined on the basis of microscopic examination, biochemical testing, and fatty acid analysis was 95%; however, MLSA was more discriminatory. Nocardia cyriacigeorgica (21.58%) and N. farcinica (14.74%) were the most frequently encountered species among clinical isolates. In summary, five-locus MLSA is a reliable method of elucidating taxonomic data to inform Nocardia species identification; however, three-locus (gyrB-16S-secA1) or four-locus (gyrB-16S-secA1-hsp65) MLSA was nearly as reliable, correctly identifying 98.5% and 99.5% of isolates, respectively, and would be more feasible for routine use in a clinical reference microbiology laboratory.

Antimicrobial susceptibility patterns of 112 clinical isolates, 28 type strains, and 9 reference strains of Nocardia were determined using the Sensititre Rapmyco microdilution panel (Thermo Fisher, Inc.). Isolates were identified by highly discriminatory multilocus sequence analysis and were chosen to represent the diversity of species recovered from clinical specimens in Ontario, Canada. Susceptibility to the most commonly used drug, trimethoprim-sulfamethoxazole, was observed in 97% of isolates. Linezolid and amikacin were also highly effective; 100% and 99% of all isolates demonstrated a susceptible phenotype. For the remaining antimicrobials, resistance was species specific with isolates of Nocardia otitidiscaviarum, N. brasiliensis, N. abscessus complex, N. nova complex, N. transvalensis complex, N. farcinica, and N. cyriacigeorgica displaying the traditional characteristic drug pattern types. In addition, the antimicrobial susceptibility profiles of a variety of rarely encountered species isolated from clinical specimens are reported for the first time and were categorized into four additional drug pattern types. N ocardia species are a group of filamentous, branching, Grampositive, modified-acid-fast bacilli that normally exist as soil saprophytes but can cause disease in immunosuppressed and healthy individuals (1). Most infections involve inhalation of fragments of filaments, resulting in pulmonary nocardiosis and pneumonia, which can be followed by dissemination to the heart, skin, subcutaneous tissue, and central nervous system (1).Nocardia taxonomy has been linked to specific patterns of antimicrobial susceptibility ever since the foundational work by Wallace et al. (2) (2, 4). With the clinical application of DNA sequencing, Nocardia taxonomy has changed and expanded rapidly. Previously identified species have been reclassified as species complexes encompassing multiple species, and numerous novel Nocardia species have been identified (5). Currently, 87 species are enumerated in the List of Prokaryotic Names with Standing in Nomenclature (LPSN) (http://www.bacterio.net/nocardia.html), many of which are clinically significant (1).However, data on antimicrobial susceptibility has lagged behind the advances in taxonomy, with only a few reports providing recent data on newer antimicrobials (4,6,7,8,9,10,11,12,13). These reports largely focus on the species traditionally associated with clinical infections, such that antimicrobial susceptibility data are not available for a large number of newly identified, but clinically relevant, Nocardia species that are isolated less frequently in the clinical laboratory. Furthermore, the species in these studies were identified using microscopy and biochemical testing, or the identification relied heavily on the 16S rRNA gene sequence analysis, both of which are unable to reliably discriminate many Nocardia species (5,14,15).The purpose of this study was to profile the antimicrobial susceptibility patterns of a diverse range of Nocardia species isolated from clinica...

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