Cucurbitacin B interacts synergistically with antibiotics against Staphylococcus aureus clinical isolates and exhibits antiviral activity against HSV-1

The search for biologically promising compounds from natural sources against microbial diseases remains an important theme in drug discovery to overcome problems with drug-resistant strains. In this study, Cucurbitacin B (Cuc B), an active constituent of Ecballium elaterium L., has been investigated in vitro for its synergy effect with antibiotics against clinical isolates of Staphylococcus aureus ( S. aureus ) and anti-HSV-1 activity. Broth microdilution method was used to determine the antibacterial activity, while checkerboard assay was used to evaluate the synergy effect according to ΣFIC indices. The anti-HSV-1 activity was determined by the plaque number reduction assay, while cytotoxicity was evaluated by MTT assay. In this study, Cuc B exerted direct growth-inhibitory activity against all S. aureus strains tested with MICs values ranging from 0.12 to 0.44 μg/mL, as well as synergy effect with tetracycline or oxacillin against four of six S. aureus strains tested (ƩFIC ranging from 0.29 to 0.43). Cuc B showed remarkable anti-HSV-1 activity compared with that of acyclovir with IC 50 values of 0.94 and 1.74 μM, respectively and selectivity indices SI = 127.7 and SI > 132.2, respectively. This study presents Cuc B as a promising therapeutic agent in the development of anti-staphylococcal and anti-HSV-1 drugs.