In vitro susceptibility of Cutibacterium acnes to dalbavancin: comparison between gradient test and the agar dilution reference method.

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.