Antimicrobial resistance patterns of Nocardia species in clinical isolates from Argentina (2020-2022)

Nocardia species are opportunistic pathogens with intrinsic antimicrobial resistance and regional variability in susceptibility patterns. Argentina lacks standardized surveillance data, complicating empirical therapy. This study aimed to characterize Nocardia species distribution and antimicrobial resistance profiles nationwide.

We analyzed 62 clinical Nocardia isolates from 41 laboratories across eight Argentine provinces (2020-2022). Species identification utilized MALDI-TOF mass spectrometry. Antimicrobial susceptibility testing was performed via broth microdilution for 11 antibiotics according to CLSI guidelines, with additional E-test comparisons.

Respiratory specimens predominated (67.7%), followed by central nervous system (14.5%), skin/soft tissue (8.1%), and bloodstream (6.5%) infections. Primary species identified were N. cyriacigeorgica (30.6%), N. abscessus complex (22.6%), and N. farcinica complex (17.7%). Highest susceptibility rates occurred with linezolid (100%) and trimethoprim-sulfamethoxazole (97.6%). Significant resistance was observed for clarithromycin (81.0%), ciprofloxacin (76.2%), and moxifloxacin (57.1%). N. farcinica dominated CNS infections (55.6%), while N. brasiliensis complex prevailed in skin/soft tissue cases (80%). Methodological discrepancies between testing methods showed 33.3% major errors for ceftriaxone (E-test vs. microdilution).

This first Argentine nationwide surveillance study confirms trimethoprim-sulfamethoxazole and linezolid as reliable empiric therapy options while highlighting concerning resistance to fluoroquinolones and macrolides. The observed methodological discordance emphasizes the critical need for standardized antimicrobial susceptibility testing protocols for Nocardia species.