Broth Microdilution and Gradient Diffusion Strips vs. Reference Agar Dilution Method: First Evaluation for...

Area of Science:

• Clinical Microbiology
• Infectious Diseases
• Antimicrobial Resistance

Background:

• Antimicrobial susceptibility testing (AST) for anaerobic bacteria presents significant challenges.
• Standardized methods like broth microdilution and diffusion strips are crucial for guiding treatment, yet their reliability needs rigorous assessment.
• Current evaluations often lack comparison against established reference methods.

Purpose of the Study:

• To evaluate the performance of two commercial broth microdilution kits and one gradient diffusion strip method for anaerobic bacteria AST.
• To compare these methods against the agar dilution reference method for key antimicrobial agents.
• To assess analytical performance according to Food and Drug Administration (FDA) guidelines.

Main Methods:

• Evaluation of two broth microdilution kits (MICRONAUT-S Anaerobes® MIC, Sensititre Anaerobe MIC®) and one gradient diffusion strip (Liofilchem®).
• Testing of 47 Clostridiales isolates (Clostridium, Clostridioides, Hungatella species).
• Comparison against the agar dilution reference method for six antimicrobial molecules, assessing essential agreement (EA) and categorical agreement (CA).

Main Results:

• Essential and categorical agreements varied significantly across methods and antimicrobial agents.
• Vancomycin demonstrated high agreement (above 90% EA and CA) across all three tested methods.
• Metronidazole, piperacillin/tazobactam, and vancomycin achieved categorical agreement meeting FDA criteria for Gram-positive anaerobic infections.
• Clindamycin exhibited the highest error rates among the tested agents.

Conclusions:

• Commercial AST methods show potential but exhibit variable performance for anaerobic bacteria.
• Vancomycin, metronidazole, and piperacillin/tazobactam appear reliable for testing key anaerobic pathogens.
• Further multicenter studies are essential to validate these findings and establish robust AST protocols for anaerobes.