Susceptibility profiles of Nocardia isolates based on current taxonomy

Area of Science:

• Medical Microbiology
• Infectious Diseases
• Antimicrobial Resistance

Background:

• The genus Nocardia is expanding, with new species emerging as human pathogens.
• Established Nocardia species have predictable antimicrobial susceptibility, but data for new species are often lacking.
• The efficacy of sulfonamides, a common first-line treatment, is increasingly questioned for Nocardia infections.

Purpose of the Study:

• To review antimicrobial susceptibility patterns for a large cohort of molecularly identified Nocardia clinical isolates.
• To assess resistance trends across various Nocardia species, including newly described ones.
• To inform clinical decision-making regarding Nocardia treatment strategies.

Main Methods:

• Analysis of susceptibility results from 1,299 Nocardia isolates spanning 39 species/complexes over six years.
• Molecular identification of all clinical isolates.
• Testing against a panel of common antimicrobial agents.

Main Results:

• All isolates were susceptible to linezolid.
• Trimethoprim-sulfamethoxazole (TMP-SMX) resistance was rare (2%) but significant in Nocardia pseudobrasiliensis (31%) and the N. transvalensis complex (19%).
• High rates of resistance to multiple common drugs were observed in N. pseudobrasiliensis (100%), N. transvalensis complex (83%), and N. farcinica (68%).

Conclusions:

• Antimicrobial resistance in Nocardia is species-dependent and requires careful consideration.
• While some resistance patterns are predictable, susceptibility testing is recommended for certain species, combination therapies, and patients with TMP-SMX intolerance.
• Linezolid demonstrates universal susceptibility among the studied Nocardia isolates.