Nocardia antimicrobial susceptibility testing by microdilution: evaluation of ready-to-use plate including tedizolid...

Nocardiosis is a rare but serious infection occurring predominantly in immunocompromised patients. The CLSI microdilution antimicrobial susceptibility testing is recommended for all nocardioses. Ready-to-use plates are available (Sensititre™ RAPMYCOI), but antimicrobials of interest (e.g. tedizolid, meropenem) are lacking. At the request of the French nocardiosis reference medical biology laboratory, a plate including meropenem and tedizolid was produced (Sensititre™ FRNOCAR1).

FRNOCAR1 was compared to RAPMYCOI using 133 clinical Nocardia isolates containing predominantly N. farcinica (23.3%), N. cyriacigeorgica (24.8%), and N. nova complex (21.1%). Acceptability was assessed using the FDA criteria (essential/category agreement; major, and very major error [VME] rates).

All criteria were respected for 6 antimicrobials; for 5, the upper limit of the VME rate exceeded the criteria, despite a point estimate of 0.0%, due to a lack of resistant strains in the collection. For linezolid, the VME rate could not be calculated due to the absence of linezolid-resistant strains. Tedizolid minimal inhibitory concentration (MIC)50/MIC90 were 2-3 dilutions lower than that of linezolid (0.5/0.5 µg/mL and 2/4 µg/mL, respectively). Imipenem was better than meropenem for N. farcinica; meropenem MIC was at least 2-dilutions lower than that of imipenem for N. gamkensis (8/8), N. gipuzkoensis (2/4), and N. wallacei (4/6).

For the antimicrobials common to both plates, the FRNOCAR1 demonstrated good agreement compared with RAPMYCOI. FRNOCAR1 includes tedizolid and meropenem, for which MIC could be reported upon justified request, with a comment indicating that these results are pending further validation.