Activity of cloxacillin against Cutibacterium acnes: MIC evaluation and utility of gradient test.

BACKGROUND

Cutibacterium acnes is a cause of implant-associated infections, often co-occurring with methicillin-susceptible Staphylococcus aureus (MSSA), especially in orthopedic settings. In Scandinavia, cloxacillin may be used as initial empiric intravenous treatment if MSSA implant-associated infection is suspected. Clinical breakpoints for C. acnes susceptibility to cloxacillin or other isoxazolyl-penicillins are lacking to guide therapy if C. acnes is subsequently cultured after empiric cloxacillin therapy has been initiated.

OBJECTIVE

We aimed to compare MIC distributions of C.acnes to benzylpenicillin and cloxacillin. Furthermore, we investigated if the gradient test is a reliable method for determining cloxacillin susceptibility in clinical C. acnes isolates in order to manage mixed infections in orthopedic settings.

METHODS

We evaluated the in vitro susceptibility of 39 clinical C. acnes isolates to benzylpenicillin and cloxacillin using gradient tests and validated cloxacillin MICs by agar dilution.

RESULTS

All isolates were susceptible to benzylpenicillin (MIC ≤0.064 mg/L) and cloxacillin (MIC ≤0.5 mg/L), with MIC50 values of 0.016 mg/L and 0.125 mg/L, respectively. The cloxacillin MIC distribution for C. acnes closely resembled that of S. aureus, suggesting that susceptibility to cloxacillin may be inferred from benzylpenicillin susceptibility. Gradient test results for cloxacillin showed high agreement with agar dilution (essential agreement 0.87), supporting its use as a practical alternative to agar dilution cloxacillin MIC determination if susceptibility testing is necessary.

CONCLUSION

Our findings indicate that intravenous cloxacillin is an effective treatment of C. acnes infections, and that cloxacillin gradient testing is a convenient and reliable alternative for MIC determination for clinical laboratories.