Note: Referral of isolates to a reference laboratory, such as a state health department laboratory or CDC’s Special Bacteriology Reference Laboratory may be needed for identification and antimicrobial susceptibility testing of aerobic actinomycetes. The Clinical and Laboratory Standards Institute (CLSI) has published guidelines M24 and M62 for standardized methods and interpretations of antimicrobial testing of the aerobic actinomycetes.61,62
Methods
- The recommended method for susceptibility testing of Nocardia, Actinomadura, Tsukamurella, Gordonia, Streptomyces, and Rhodococcus is the broth microdilution method.63
- A disc diffusion test using sulfisoxazole may be used to help with sulfonamide drug susceptibility interpretation.
- Some research has indicated E-test may provide better results than disc diffusion.63
Antibiotics to test include the following63,64:
- Primary profile: amikacin, amoxicillin-clavulanate, ceftriaxone, ciprofloxacin, clarithromycin, imipenem, linezolid, minocycline, moxifloxacin, sulfamethoxazole or trimethoprim-sulfamethoxazole, and tobramycin
- Secondary profile: cefepime, cefotaxime, and doxycycline
Limitations63
- Proper inoculum preparation is important as these organisms are typically difficult to emulsify.
- Susceptibility results that do not match expected patterns should be reviewed, and testing repeated or sent to another laboratory for confirmation.
- False-resistant results may occur when testing ceftriaxone against N. brasiliensis using the MIC method.
- False-resistant results may occur when testing imipenem against N. farcinica using the MIC method.
61. Clinical and Laboratory Standards Institute (CLSI). Performance standards for susceptibility testing of Mycobacteria, Nocardia spp., and other aerobic actinomycetes. CLSI supplement M62. Updated February 2019, March 2019, and August 2020. Wayne, PA: Clinical and Laboratory Standards Institute; 2018.62. Clinical and Laboratory Standards Institute (CLSI). Susceptibility testing of Mycobacteria, Nocardia spp., and other aerobic actinomycetes. CLSI document M24. Updated October 2019, and April 2020. Wayne, PA: Clinical and Laboratory Standards Institute; 2018.
63. Leber AL, ed. Clinical microbiology procedures handbook. 4th ed. ASM Press; 2016:Section 6. 64. Mochon AB, Sussland D, Saubolle MA. Aerobic actinomycetes of clinical significance. Microbiol Spectr. 2016;4(4):10.1128/microbiolspec.DMIH2-0021-2015. doi:10.1128/microbiolspec.DMIH2-0021-2015
