Species distribution and antimicrobial susceptibility of Nocardia isolates from patients in Iran with suspected tuberculosis: a two-year study.

INTRODUCTION

While nocardiosis can clinically and radiologically resemble tuberculosis (TB), its frequency and species-specific antimicrobial susceptibility in TB-suspected patients in Iran have yet to be conclusively characterized.

OBJECTIVES

To describe the frequency, species distribution, and in vitro antimicrobial susceptibility of Nocardia isolates retrieved from unique patients investigated for suspected TB at a referral center in Iran from March 2021 to March 2023.

METHODS

We analyzed specimens from 2000 unique patients evaluated for suspected TB. Nocardia species were identified by sequencing 16S rRNA, secA1, and rpoB. was used to assess Amikacin, linezolid, imipenem, amoxicillin-clavulanate, moxifloxacin, and trimethoprim-sulfamethoxazole (TMP-SMX) were assessed using broth microdilution.

RESULTS

Pulmonary TB was identified in 82 patients (4.1%) and TB-Nocardia coinfection in six (10.7%). The most frequent species were N. cyriacigeorgica (20/56, 35.7%), N. farcinica (12/56, 21.4%), and N. asteroides (10/56, 17.9%). Sputum (32/56, 57.1%) and bronchoalveolar lavage (13/56, 23.2%) were the most common specimen sources. Susceptibility to TMP- SMX (56/56, 100%), linezolid (56/56, 100%), and amikacin (52/56, 93%) was highest. Imipenem susceptibility varied by species; 13/56 isolates (23%) were susceptible to moxifloxacin, and 20/56 (36%) to amoxicillin-clavulanate. These are in vitro findings; treatment outcomes were not assessed.

CONCLUSION

In a single-center cohort of patients investigated for suspected TB but without confirmed TB or another mycobacterial infection, Nocardia was detected in 2.5%. Even though species identification and isolate-specific susceptibility testing may help to inform clinical assessment, the findings require validation and do not conclusively establish clinical treatment effectiveness or failure.