Susceptibility Profiles of Nocardia Isolates Based on Current Taxonomy

The genusNocardia has undergone rapid taxonomic expansion in recent years, and an increasing number of species are recognized as human pathogens. Many established species have predictable antimicrobial susceptibility profiles, but sufficient information is often not available for recently described organisms. Additionally, the effectiveness of sulfonamides as first-line drugs forNocardia has recently been questioned. This led us to review antimicrobial susceptibility patterns for a large number of molecularly identified clinical isolates. Susceptibility results were available for 1,299 isolates representing 39 different species or complexes, including 11 that were newly described, during a 6-year study period. All tested isolates were susceptible to linezolid. Resistance to trimethoprim-sulfamethoxazole (TMP-SMX) was rare (2%) except amongNocardia pseudobrasiliensis (31%) strains and strains of theN. transvalensis complex (19%). Imipenem susceptibility varied forN. cyriacigeorgica andN. farcinica , as did ceftriaxone susceptibility of theN. nova complex. Resistance to more than one of the most commonly used drugs (amikacin, ceftriaxone, TMP-SMX, and imipenem) was highest forN. pseudobrasiliensis (100%),N. transvalensis complex (83%),N. farcinica (68%),N. puris (57%),N. brasiliensis (51%),N. aobensis (50%), andN. amikacinitolerans (43%). Thus, while antimicrobial resistance can often be predicted, susceptibility testing should still be considered when combination therapy is warranted, for less well characterized species or those with variable susceptibility profiles, and for patients with TMP-SMX intolerance.