Synergistic Effect and Time-Kill Evaluation of Eugenol Combined with Cefotaxime Against Staphylococcus aureus

Eugenol, a clove-derived aromatic compound has shown antibacterial activity against many species, including Staphylococcus aureus. Epidemiology studies from the past two decades reported an increased number of healthcare-associated and skin tissue infections due to S. aureus antimicrobial resistance (AMR) including several cases of resistance to β-lactam antibiotics, such as cefotaxime. We aimed to investigate whether eugenol can cause lethality of S. aureus including the strain resistant to methicillin and the wild strain isolated from a hospital patient. Moreover, we asked whether eugenol could enhance the therapeutic effect of cefotaxime, one of the most prescribed 3rd generation cephalosporin β-lactam antibiotics, of which S. aureus resistance to this antibiotic has emerged. The minimum inhibitory concentration (MIC) of each substance was determined using the standard broth microdilution test following the combination experiment performed using checkerboard dilution. The type of interactions, including synergistic and additivity, was determined using isobologram analysis, and the dose reduction index (DRI) was calculated. The time-kill kinetic assay was performed to evaluate the dynamic bactericidal activity of eugenol alone and in combination with cefotaxime. We showed that eugenol alone is bactericidal against S. aureus ATCC 33591 and the clinical isolate. Eugenol combined with cefotaxime resulted synergistic effect against S. aureus ATCC 33591, ATCC 29213, and ATCC 25923. Eugenol may be capable to improve the therapeutic effect of cefotaxime against methicillin-resistant S. aureus (MRSA).