In vitro activity of omadacycline against clinical isolates of Nocardia

Abstract

Nocardiosis typically requires a prolonged treatment duration of ≥6 months and initial combination therapy with 2-3 antibiotics. First-line regimens for nocardiosis are associated with considerable toxicity; therefore, alternative therapies are needed. Omadacycline is an aminomethylcycline with broad antimicrobial activity whose in vitro activity against Nocardia species has not been formally assessed. The in vitro potency of omadacycline was evaluated against 300 Nocardia clinical isolates by broth microdilution. The most common Nocardia species tested were N. cyriacigeorgica (21%), N. nova (20%), and N. farcinica (12%). The most common specimens were respiratory (178 isolates, 59%) and wound (57 isolates, 19%). Omadacycline minimum inhibitory concentrations (MICs) across all Nocardia species ranged from 0.06 µg/mL to 8 µg/mL, with an MIC₅₀ of 2 µg/mL and MIC₉₀ of 4 µg/mL. The lowest MICs were found among N. paucivorans (MIC₅₀ = 0.25 µg/mL, MIC₉₀ = 0.25 µg/mL), N. asiatica (MIC₅₀ = 0.25 µg/mL, MIC₉₀ = 1 µg/mL), N. abscessus complex (MIC₅₀ = 0.5 µg/mL, MIC₉₀ = 1 µg/mL), N. beijingensis (MIC₅₀ = 0.5 µg/mL, MIC₉₀ = 2 µg/mL), and N. otitidiscaviarum (MIC₅₀ = 1 µg/mL, MIC₉₀ = 2 µg/mL). The highest MICs were found among N. farcinica (MIC₅₀ = 4 µg/mL, MIC₉₀ = 8 µg/mL). In vitro potency differed by species among Nocardia clinical isolates. Further studies are warranted to evaluate the potential clinical utility of omadacycline for nocardiosis.

Keywords: Nocardia; antimicrobial susceptibility; omadacycline.

Fig 1

Distribution of omadacycline and tigecycline MICs against reference QC strain N. nova ATCC BAA-2227. Because QC ranges have not been established for tigecycline or omadacycline, we performed 18 independent replicate reference BMD tests using the CLSI Nocardia nova QC organism. All MICs were 1–2 µg/mL for tigecycline and 2–4 µg/mL for omadacycline.