Population-based microbiological characterization of Nocardia strains causing invasive infections during a multiyear period in a large Canadian healthcare region.

The role of specific Nocardia species in human infections continues to expand as advanced methods enable species/species complex identification. We conducted a multiyear population-based (2010-2022) characterization of invasive Nocardia spp. Isolates were analyzed using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) and 16S rRNA gene sequencing. A total of 94 cases and 117 unique 16S rRNA gene sequences were evaluated from pulmonary (53%), skin and soft tissues (SSTIs) (29%), central nervous system (CNS) (7%), bloodstream (6%), and other sites of infection. Infections were mainly caused by three species complexes: N. farcinica (n = 20, 21.3%), N. cyriacigeorgica (n = 16, 17%), and N. nova (n = 15, 16%). Phylogenetic analysis correlated with the clinical site of infection. Most N. cyriciageorgica (92%) complex isolates caused SSTIs or pulmonary infections, and most N. farcinica complex (67%) and N. nova complex (69%) isolates caused pulmonary and CNS infections, respectively. Several other unique Nocardia spp. rarely caused invasive infections (≤5 cases): (i) pulmonary (N. abscessus complex, N. amamiensis, N. asteroides, N. otitidiscaviarum complex, N. sputorum, N. transvalensis complex, Nocardiopsis spp., Pseudonocardia spp.); (ii) SSTIs (N. otitidiscaviarum complex); and (iii) CNS (N. flavorosea and N. paucivorans complex). Nocardia species were highly susceptible to amikacin, trimethoprim-sulfamethoxazole, moxifloxacin, and linezolid. Imipenem resistance occurred in the N. farcinica complex and N. nova complex, while ceftriaxone resistance only occurred in the former. Antibiotic profiles varied for rare Nocardia spp. Species-level Nocardia identification using MALDI-TOF MS and 16S rRNA gene sequencing improves understanding of these organisms' unique roles in causing invasive disease.IMPORTANCENocardia spp. are a rare cause of invasive infections, particularly in immunocompromised patients. The role of specific Nocardia species in human infections continues to expand as advanced methods enable species/species complex identification. We conducted a multiyear population-based (2010-2022) characterization of invasive Nocardia spp. Isolates were analyzed using matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) and 16S rRNA gene sequencing. A total of 94 cases included pulmonary infections (53%), skin and soft tissue infections (29%), central nervous system infections (7%), bloodstream infections (6%), and septic arthritis and intra-abdominal infections (5%). One hundred seventeen unique 16S rRNA gene sequences from clinical isolates were analyzed. Phylogenetic analysis correlated with the clinical site of infection. Nocardia species were highly susceptible to amikacin, trimethoprim-sulfamethoxazole, moxifloxacin, and linezolid. Imipenem resistance only occurred in N. farcinica complex and N. nova complex strains, and ceftriaxone resistance only occurred in the former. Species-level Nocardia identification using MALDI-TOF MS and 16S sequencing improves understanding of these organisms' unique roles in invasive disease.