Antimicrobial Susceptibility among Clinical Nocardia Species Identified by Multilocus Sequence Analysis

Abstract

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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