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Updated: Oct 25, 2026
A Reference Broth Microdilution Method for Dalbavancin In Vitro Susceptibility Testing of Bacteria that Grow Aerobically
Published on: September 9, 2015
In vitro susceptibility of Cutibacterium acnes to dalbavancin: comparison between gradient test and the agar dilution
Sara Svensson1,2, Susanne Jacobsson3, Erika Matuschek4
1School of Medical Sciences, Faculty of Medicine and Health, Örebro University, Örebro, Sweden.
Background:
Cutibacterium acnes, a human commensal, occasionally causes infections requiring long-term antibiotic treatment, including periprosthetic joint infections. Dalbavancin is a new lipoglycopeptide that allows intermittent dosing because of its exceptionally long half-life. However, data on susceptibility of C. acnes to dalbavancin are lacking, and no EUCAST breakpoints have been defined.
Objectives:
To determine the MIC distribution of C. acnes for dalbavancin, and to compare the gradient test to the agar dilution reference method.
Methods:
C. acnes isolates were obtained from tissue samples from arthroplasties (n = 117), skin samples (n = 15), and positive blood cultures (n = 85). Antibiotic susceptibility was tested by agar dilution according to EUCAST methodology for anaerobic bacteria (±polysorbate 80) and by gradient test.
Results:
MIC50 and MIC90 for dalbavancin against C. acnes were 0.03 and 0.06 mg/L using agar dilution, and 0.004 and 0.008 mg/L with the gradient test. The gradient test yielded 3-fold lower MIC values than agar dilution. No significant difference was observed when polysorbate 80 was added to the agar dilution test. When vancomycin was tested as a comparator, the MICs did not differ between the agar dilution and gradient tests; MIC50 and MIC90 were 0.5 mg/L in both tests.
Conclusions:
C. acnes showed significantly lower MIC values for dalbavancin than for vancomycin, highlighting the potential of dalbavancin as a therapeutic option. The gradient test yielded significantly lower MICs for dalbavancin than the reference method for anaerobes, agar dilution, suggesting that the gradient test for dalbavancin may not be a reliable method for antibiotic susceptibility testing (AST) of C. acnes.
