Evaluation of Alamar colorimetric broth microdilution susceptibility testing method for staphylococci and enterococci

Area of Science:

• Clinical Microbiology
• Infectious Diseases
• Antimicrobial Resistance

Background:

• Accurate antimicrobial susceptibility testing (AST) is crucial for guiding antibiotic therapy and combating resistance.
• Traditional broth microdilution methods are reliable but can be labor-intensive.
• Novel, user-friendly AST methods are needed to improve efficiency in clinical laboratories.

Purpose of the Study:

• To evaluate the performance of the Alamar colorimetric broth microdilution method for AST.
• To compare Alamar method results against the National Committee for Clinical Laboratory Standards (NCCLS) reference method.
• To assess the accuracy of Alamar for testing Gram-positive organisms, including staphylococci and enterococci.

Main Methods:

• Compared Alamar broth microdilution with NCCLS reference broth microdilution for 119 Gram-positive bacterial isolates.
• Tested susceptibility to 20 antimicrobial agents appropriate for each bacterial species.
• Analyzed absolute categorical agreement and essential agreement between the two methods.

Main Results:

• High absolute categorical agreement: 91.5% for enterococci, 99.8% for oxacillin-susceptible staphylococci, and 97.4% for oxacillin-resistant staphylococci.
• Essential agreement exceeded 99% for all tested organisms.
• Minimal errors observed, with most minor errors within one log2 dilution of reference results; Alamar system was easy to use and interpret.

Conclusions:

• The Alamar colorimetric system demonstrates acceptable accuracy for AST of staphylococci and enterococci.
• The method offers a user-friendly alternative to traditional reference AST methods.
• Alamar provides reliable susceptibility data for guiding treatment of Gram-positive bacterial infections.