Phenotypic and genotypic profiles of clinical isolates of various Nocardia species to carbapenems and fluoroquinolones.

Objectives: To establish patterns of the antimicrobial susceptibility of Nocardia species to carbapenems and fluoroquinolones and analysis of phenotypic-genotypic correlations.

Methods: Isolates were identified to the species using 16S rRNA, secA1, or rpoB gene sequencing analysis. The antimicrobial susceptibility testing was performed using the broth microdilution method, and WGS was employed to analyse the presence of resistance genes and/or mutations of Nocardia species against carbapenems and fluoroquinolones.

Results: Among 143 Nocardia isolates, N. farcinica (27.27%, 39/143) and N. cyriacigeorgica (25.17%, 36/143) were the most common species, followed by N. abscessus Complex (18.88%, 27/143). The MIC90s of the seven carbapenems were 8 mg/L for doripenem, 8 mg/L for meropenem, 16 mg/L for ertapenem, 16 mg/L for biapenem, 64 mg/L for imipenem, 64 mg/L for faropenem and 128 mg/L for tebipenem, respectively. The susceptibility rates to imipenem were 76.9% and 88.9% for N. farcinica and N. cyriacigeorgica, respectively, but only 14.3% and 0% for N. otitidiscavarium and N. brasiliensis, respectively. Further, 90% of N. brasiliensis and 50% of N. otitidiscaviarum isolates were susceptible and intermediate to meropenem. WGS identified blaFAR-1 gene in N. farcinica and blaAST-1 gene in N. cyriacigeorgica, respectively. The MIC90s of the four fluoroquinolones were 1 mg/L for sitafloxacin, 4 mg/L for nemonoxacin, 4 mg/L for moxifloxacin and 16 mg/L for ciprofloxacin, respectively. The susceptibility rate of Nocardia species to ciprofloxacin was low except for N. farcinica. The resistance to fluoroquinolones arise from mutations in the gyrA gene.

Conclusions: Nocardia spp. exhibited varying patterns of susceptibility to carbapenems and fluoroquinolones respectively. Importantly, different Nocardia spp. exhibited different patterns of susceptibility to carbapenems and fluoroquinolones, respectively.